input: rework mouse button/motion handling

Store a list of currently pressed buttons, and which surface they
belong to (i.e. which surface that received the press).

Then, in motion events (with a button pressed, aka drag operations),
send the event to the “original” surface (that received the press).

Also send release events to the originating surface.

This means a surface receiving a press will always receive a
corresponding release. And no one will receive a release event without
a corresponding press event.

Motion events with a button pressed will *always* use the *first*
button that was pressed. I.e. if you press a button, start dragging,
and then press another button, we keep generating motion events for
the *first* button.

selection.c

@@ -634,6 +634,8 @@ selection_finalize(struct seat *seat, struct terminal *term, uint32_t serial)
     if (!term->selection.ongoing)
         return;
 
+    LOG_DBG("selection finalize");
+
     selection_stop_scroll_timer(term);
     term->selection.ongoing = false;