osd: make window switcher more Openbox-like in terms of key precessing

Before this commit, keystrokes were interpreted based on following
hard-coded rules while the window switcher is active:

  1. Up/Left arrow keys cycle the window forward.
  2. Down/Right arrow keys cycle the window backward.
  3. Other keystrokes cycle the window in the initial direction specified
     by NextWindow/PreviousWindow actions. But while Shift key is pressed,
     the direction is inverted.

...and keybind actions were never executed.

However, this lead to a counter-intuitive behavior for new, especially
pre-Openbox users. For example, in the following keybinds, after the user
activates the window switcher with Super+n, Super+p cycles the window
_forward_:

  <keybind key="W-n">
    <action name="NextWindow" />
  </keybind>
  <keybind key="W-p">
    <action name="PreviousWindow" />
  </keybind>

This is because the key 'n' is recognized just as a normal key in the
third hard-coded rule.

So this commit changes the rules to be more Openbox-like:

  1. Up/Left arrow keys cycles the window forward.
  2. Down/Right arrow keys cycles the window backward.
  3. Other keystrokes are matched against keybinds and execute their
     actions. If they include NextWindow/PreviousWindow action, it cycles
     the selected window forward/backward even while the window switcher
     is active.

include/labwc.h

@@ -387,8 +387,6 @@ struct server {
 		struct wlr_scene_tree *preview_parent;
 		struct wlr_scene_node *preview_anchor;
 		struct multi_rect *preview_outline;
-		enum lab_cycle_dir initial_direction;
-		bool initial_keybind_contained_shift;
 	} osd_state;
 
 	struct theme *theme;