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

include/sway/tree/container.h

@@ -304,6 +304,12 @@ bool container_is_floating(struct sway_container *container);
  */
 void container_get_box(struct sway_container *container, struct wlr_box *box);
 
+/**
+ * Move a floating container by the specified amount.
+ */
+void container_floating_translate(struct sway_container *con,
+		double x_amount, double y_amount);
+
 /**
  * Move a floating container to a new layout-local position.
  */
@@ -318,4 +324,10 @@ void container_set_dirty(struct sway_container *container);
 
 bool container_has_urgent_child(struct sway_container *container);
 
+/**
+ * If the container is involved in a drag or resize operation via a mouse, this
+ * ends the operation.
+ */
+void container_end_mouse_operation(struct sway_container *container);
+
 #endif

include/sway/tree/layout.h

@@ -14,10 +14,11 @@ enum movement_direction {
 };
 
 enum resize_edge {
-	RESIZE_EDGE_LEFT,
-	RESIZE_EDGE_RIGHT,
-	RESIZE_EDGE_TOP,
-	RESIZE_EDGE_BOTTOM,
+	RESIZE_EDGE_NONE   = 0,
+	RESIZE_EDGE_LEFT   = 1,
+	RESIZE_EDGE_RIGHT  = 2,
+	RESIZE_EDGE_TOP    = 4,
+	RESIZE_EDGE_BOTTOM = 8,
 };
 
 struct sway_container;