Implement floating_modifier and mouse operations for floating views

This implements the following:

* `floating_modifier` configuration directive
* Drag a floating window by its title bar
* Hold mod + drag a floating window from anywhere
* Resize a floating view by dragging the border
* Resize a floating view by holding mod and right clicking anywhere on
the view
* Resize a floating view and keep aspect ratio by holding shift while
resizing using either method
* Mouse cursor turns into resize when hovering floating border or corner

sway/tree/view.c

@@ -387,6 +387,8 @@ void view_set_fullscreen(struct sway_view *view, bool fullscreen) {
 		}
 	}
 
+	container_end_mouse_operation(view->swayc);
+
 	ipc_event_window(view->swayc, "fullscreen_mode");
 }