feat(wayland): touch, with the first finger standing in for the pointer

The seat's wl_touch was never bound, so a touchscreen could not drive a
window at all. Every finger is now reported through onTouchDown, onTouchMove,
onTouchUp and onTouchCancel (TouchPoint: the compositor's id and the
position in physical pixels). The first finger down also enters the window —
Device::focusedWindow and the pointer serial the clipboard needs — presses
the left button, moves it and releases it when it lifts, so an application
written for a pointer works on a phone as it is.

interfaces/Crafter.Graphics-Types.cppm

@@ -18,6 +18,13 @@ export namespace Crafter {
         Vector<float, 2> mouseDelta;
     };
 
+    // One finger on a touchscreen, in the window's physical pixels; the id is
+    // the compositor's and stays with the finger until it lifts.
+    struct TouchPoint {
+        std::int32_t id;
+        Vector<float, 2> position;
+    };
+
     struct ScaleData2D {
         Vector<std::int16_t, 2, 2> position;
         Vector<std::int16_t, 2, 2> size;