Make a folder from the server browser

The in-app browser that chooses a library folder on the server could
only open folders that already existed, so a library, or an export
destination, that was not on the server yet had to be made in the
Nextcloud web page first. It now has "New folder": a name, then MKCOL,
then the parent listed again and the new folder walked into — a folder
somebody has just named is the one they mean to choose.

The listing is the server's rather than the name inserted locally: the
server may have normalised or refused it. A name with a slash, "..",
or nothing at all is refused before any request, because a folder
typed with a slash in it is a path the user did not mean.

remote_folders holds the two WebDAV round trips (list, make) off the UI
thread, with the answer delivered through a Slint timer, so the album
sheet can use the same browser.

ui/dr-ui/ui/app.slint

@@ -320,6 +320,7 @@ export component AppWindow inherits Window {
     callback launch-browse-up();
     callback launch-browse-confirm();
     callback launch-browse-cancel();
+    callback launch-browse-make(string);
 
     // --- library grid (FR-CAT-4) ---
     //
@@ -1374,6 +1375,7 @@ in property <bool> panel-visible: true;
             browse-up() => { root.launch-browse-up(); }
             browse-confirm() => { root.launch-browse-confirm(); }
             browse-cancel() => { root.launch-browse-cancel(); }
+            browse-make(name) => { root.launch-browse-make(name); }
         }
 
         // The library view sits between the launch screen and develop: a library

ui/dr-ui/ui/launch.slint

@@ -124,6 +124,11 @@ export component LaunchScreen inherits Rectangle {
     callback browse-up();
     callback browse-confirm();
     callback browse-cancel();
+    /// Make a folder here, by name, and go into it.
+    callback browse-make(string);
+
+    // Whether the picker is asking for a new folder's name.
+    property <bool> naming-folder: false;
 
     background: Theme.ground;
 
@@ -393,10 +398,51 @@ export component LaunchScreen inherits Rectangle {
                     }
 
                     // Current location, so it is always clear what
-                    // "Use this folder" would select.
-                    Value {
-                        text: root.browse-path == "" ? "/" : "/" + root.browse-path;
-                        overflow: elide;
+                    // "Use this folder" would select — and beside it the way
+                    // to make one, for a library that is not on the server
+                    // yet.
+                    HorizontalLayout {
+                        spacing: Theme.gap;
+                        Value {
+                            text: root.browse-path == "" ? "/" : "/" + root.browse-path;
+                            overflow: elide;
+                            horizontal-stretch: 1;
+                            min-width: 0px;
+                            vertical-alignment: center;
+                        }
+                        FormButton {
+                            text: @tr("New folder");
+                            enabled: !root.browse-loading && !root.naming-folder;
+                            clicked => { root.naming-folder = true; }
+                        }
+                    }
+
+                    if root.naming-folder: HorizontalLayout {
+                        spacing: Theme.gap;
+                        new-name := Field {
+                            horizontal-stretch: 1;
+                            label: @tr("New folder name");
+                            placeholder: @tr("Folder name");
+                            accepted(t) => {
+                                if (t != "") {
+                                    root.browse-make(t);
+                                    root.naming-folder = false;
+                                }
+                            }
+                            init => { self.take-focus(); }
+                        }
+                        FormButton {
+                            text: @tr("Make");
+                            enabled: new-name.text != "";
+                            clicked => {
+                                root.browse-make(new-name.text);
+                                root.naming-folder = false;
+                            }
+                        }
+                        FormButton {
+                            text: @tr("Cancel");
+                            clicked => { root.naming-folder = false; }
+                        }
                     }
 
                     // Not a `Panel`: this box overlays three mutually