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.

docs/labwc-actions.5.scd

@@ -117,8 +117,6 @@ Actions are used in menus and keyboard/mouse bindings.
 	Cycle focus to next/previous window respectively.++
 	Default keybind for NextWindow is Alt-Tab.
 
-	The shift key is used to reverse direction while cycling.
-
 	The arrow keys are used to move forwards/backwards while cycling.
 
 *<action name="Reconfigure" />*