Move drag_pointer() to its own source file

Move drag_pointer() and related definitions from floating.c to new file
drag_pointer.c since it's applicable not only to floating windows but
also to resizing of tiled windows.

include/drag.h

@@ -0,0 +1,55 @@
+/*
+ * vim:ts=4:sw=4:expandtab
+ *
+ * i3 - an improved dynamic tiling window manager
+ * © 2009 Michael Stapelberg and contributors (see also: LICENSE)
+ *
+ * drag.c: click and drag.
+ *
+ */
+#pragma once
+
+#include <config.h>
+
+/** Callback for dragging */
+typedef void (*callback_t)(Con *, Rect *, uint32_t, uint32_t, const void *);
+
+/** Macro to create a callback function for dragging */
+#define DRAGGING_CB(name)                                      \
+    static void name(Con *con, Rect *old_rect, uint32_t new_x, \
+                     uint32_t new_y, const void *extra)
+
+/**
+ * This is the return value of a drag operation like drag_pointer.
+ *
+ * DRAGGING will indicate the drag action is still in progress and can be
+ * continued or resolved.
+ *
+ * DRAG_SUCCESS will indicate the intention of the drag action should be
+ * carried out.
+ *
+ * DRAG_REVERT will indicate an attempt should be made to restore the state of
+ * the involved windows to their condition before the drag.
+ *
+ * DRAG_ABORT will indicate that the intention of the drag action cannot be
+ * carried out (e.g. because the window has been unmapped).
+ *
+ */
+typedef enum {
+    DRAGGING = 0,
+    DRAG_SUCCESS,
+    DRAG_REVERT,
+    DRAG_ABORT
+} drag_result_t;
+
+/**
+ * This function grabs your pointer and keyboard and lets you drag stuff around
+ * (borders). Every time you move your mouse, an XCB_MOTION_NOTIFY event will
+ * be received and the given callback will be called with the parameters
+ * specified (client, border on which the click originally was), the original
+ * rect of the client, the event and the new coordinates (x, y).
+ *
+ */
+drag_result_t drag_pointer(Con *con, const xcb_button_press_event_t *event,
+                           xcb_window_t confine_to, border_t border, int cursor,
+                           callback_t callback, const void *extra);