Enable spare keyboard layouts in Wayland
This MR enables the spare layouts functionality in Wayland as we know it from X11. Enabling this functionality was previously discussed in the following MR and is thus the next step of what was discussed there.
The spare layout functionality in short: when enabled the user has a set of main layouts, which can be toggled with the normal keyboard shortcuts (the XKB one and the alternative one). Then there is a set of one or more additional layouts, which can only be activated either by a dedicated shortcut, or by clicking on the layout in the tray icon. Once a spare layout is chosen, it replaces the last layout from the main set.
Example:
Main layouts: EN, DE, ES
Spare layouts: BG, JP
Usually the toggled layouts will be: EN -> DE -> ES -> EN -> DE -> ES -> EN, etc.
If JP gets selected the toggle will be: EN -> DE -> JP -> EN -> DE -> JP -> EN, etc.
If then BG gets selected the toggle will be: EN -> DE -> BG -> EN -> DE -> BG -> EN, etc.