Shrink to a floating button instead of hiding

With "Shrink to Button" on, the dismiss key shrinks the window to just
itself: a see-through button that can be dragged anywhere. Tapping it
grows the keyboard back around it, kept on screen. KWin moves and
resizes the window through a one-off script, as Wayland clients can't
place themselves.

Also moves the window back on screen whenever it appears (KWin remembers
where the button was), and releases the mouse after a window manager
move or resize, which used to eat the next press on the grab bar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

qml/Keyboard.qml

@@ -4,7 +4,18 @@ import Sloposk
 KeyboardSurface {
     id: surface
 
+    // Window-relative rect of the first key of `type`; empty if there is none.
+    function keyRect(type) {
+        for (let i = 0; i < keys.count; ++i) {
+            const key = keys.itemAt(i);
+            if (key?.keyType === type)
+                return key.mapToItem(null, 0, 0, key.width, key.height);
+        }
+        return Qt.rect(0, 0, 0, 0);
+    }
+
     Repeater {
+        id: keys
         model: surface.model
         delegate: Key {
             shiftMode: surface.controller ? surface.controller.shiftMode : KeyboardController.Off