Add keyboard shortcuts for general navigation following the Meta xdg spec

This is a simple but major commit as it implements default shortcuts for window movement following the new Meta spec. Those are what I suggested in https://phabricator.kde.org/T11520 that I think work as reasonable defaults.

With this commit, we have an entire set of navigation shortcuts based on Meta:

• Snapping windows = Meta + arrows (preexistent, improved by Nate)
• Switch focus from one window to another = Meta + Alt + arrows (preexistent)
• Move window to next/previous screen = Meta + Shift + arrows (this commit)
• Switch focus from one virtual desktop to another = Meta + Ctrl + arrows (preexistent, implemented by GB_2)
• Move window to another virtual desktop = Meta + Ctrl + Shift + arrows (this commit)

By using this kind of semantics where Ctrl switches focus and Shift indicates movement, if and only if we decide to implement this in the future, virtually any other keyboard shortcut based on numbers could follow the same pattern of modifier keys:

• Switch focus to virtual desktop 3 = Ctrl + F3 because it already exists, Meta + F3 for equal number of keys or Meta + Ctrl + 3 for parity
• Move window to virtual desktop 5 = Meta + Shift + F5, Meta + Shift + 5