Add toggle previous keyboard layout shortcut

This is a companion merge request to this merge request: plasma-desktop merge request 1722

This merge request enables the above functionality on Wayland. For Xorg the functionality is implemented in plasma-desktop in the above merge request.

Explanation:

Often when documents are editted, especially when users use non latin-based layouts, it's needed to quickly switch between two layouts. Users, which have more than 2 keyboard layouts have to use custom shortcuts for each layout, or have to toggle through all layouts to reach the desired one.

Using this functionality the user will always toggle between the current and the previously selected layouts, thus toggling between two concrete layouts independent of their position in the layouts list and the number of layouts configured.

@teams/qa - It's a good idea to review the code by someone from QA.