desktop-shell: shrink windows not to be obscured by input panel

This introduces the boolean option input-panel-shrink in the shell section of weston.ini.

When this option is set and an input panel becomes visible, the desktop shell checks whether any windows on screen become overlapped by the input panel. If this is the case, the heights of the affected windows are shrunk until they stop overlapping. Maximized and fullscreen windows are handled by removing their maximized or fullscreen status while the input panel is visble.

When the input panel is hidden, all affected windows are restored to their original states.

In addition a slide-out animation is added to the input panel when it is hidden to complement the existing slide-in animation when it appears.

Signed-off-by: Sebastian Urban surban@surban.net