Add toggle previous keyboard layout shortcut

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.

In this commit, I suggest a new option in the keyboard layout settings, which allows setting up a new keyboard shortcut. Using this shortcut 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.

@teams/localization - There is a new QLabel with visible text, which IMHO should be translated in all possible languages

@teams/vdg - since this is adding a new field, please review for UI

Before:

After: