Implement drag cancel for resizing floating windows

Long description:

Hi, I am a new developer and this is my first code submission to a
project. If I do anything wrong, please let me know.

The patches I've submitted implement #1074 "Add ability to escape
out of a mouse-resize operation".

To apply this patch, use:
curl http://cr.i3wm.org/patch/235/raw.patch | git am

@@ -554,7 +554,15 @@ void floating_resize_window(Con *con, const bool proportional,

     struct resize_window_callback_params params = { corner, proportional, event };

-    drag_pointer(con, event, XCB_NONE, BORDER_TOP /* irrelevant */, cursor, resize_window_callback, &params);

+    /* get the initial rect in case of cancel */

+    struct Rect initial_rect = con->rect;

+

+    int drag_result = drag_pointer(con, event, XCB_NONE, BORDER_TOP /* irrelevant */, cursor, resize_window_callback, &params);

+

+    /* If the user cancels, undo the resize */

+    if (drag_result == 1) {

+        floating_reposition(con, initial_rect);

+    }

     /* If this is a scratchpad window, don't auto center it from now on. */

     if (con->scratchpad_state == SCRATCHPAD_FRESH)