keyboard: allow applying keyboard layout per window

keyboard: allow applying keyboard layout per window

Fixes #1076

It can be enabled with a config like

~/.config/labwc/rc.xml:
<keyboard layoutScope="window">

~/.config/labwc/environment:
XKB_DEFAULT_LAYOUT=de,us
XKB_DEFAULT_OPTIONS=grp:alt_shift_toggle,grp_led:scroll

With a configuration like this each window should now remember
the active keyboard layout when switching between windows.

By default, the keyboard layout keeps being a global state.