Add configuration option for disabling mouse warping

This patch adds a new configuration option "mouse_warping [output|none]".

When mouse warping is disabled, mouse cursor does not jump to middle of current
screen when changing workspaces between multiple outputs. This introduces a
"special" cursor state, where focus is in one window and cursor on another.
Useful for eg. scrolling a web page with mouse wheel while typing into another
window on keyboard.

src/x.c

@@ -1142,7 +1142,8 @@ void x_set_i3_atoms(void) {
  */
 void x_set_warp_to(Rect *rect)
 {
-    if (!config.disable_focus_follows_mouse)
+    if (!config.disable_focus_follows_mouse &&
+        config.mouse_warping != POINTER_WARPING_NONE)
         warp_to = rect;
 }