Offer a mask that is nothing but a hand

Every route to a layer began with a selection — a gradient, a band, a subject,
a category — and painting was reachable only by making one of those and
joining a painted part to it. So the answer to "brush a correction onto this
corner of the sky" was "add a radial gradient you do not want, then paint into
that", which is not an answer.

Paint sits beside Linear and Radial and makes a layer whose base is a brush.
It covers nothing until a stroke lands in it, so pressing it arms the brush
and shows the mask as well: a row that appeared and changed no pixel, with the
pointer still in "select", is indistinguishable from a button that did nothing.

ui/dr-ui/ui/masks.slint

@@ -970,6 +970,22 @@ export component MaskPanel inherits Rectangle {
                 enabled: Develop.enabled;
                 clicked => { Masking.add-gradient(true); }
             }
+
+            // TRACES: FR-DEV-19b
+            // **A mask that is nothing but a hand.** Beside the gradients
+            // because it belongs to the same answer to "where" — and because
+            // without it the only way to reach the brush was to make a
+            // gradient nobody wanted and paint into that.
+            //
+            // It covers nothing until a stroke lands, so pressing this arms
+            // the brush as well as making the layer: a row that appeared and
+            // changed no pixel, with the pointer still in "select", would look
+            // exactly like a button that did nothing.
+            Button {
+                text: "Paint";
+                enabled: Develop.enabled;
+                clicked => { Masking.add-brush(); }
+            }
         }
 
         // Ranges, on their own row rather than beside the gradients. A