Use Meta+Alt based keyboard shortcuts for desktop switching actions

Current shortcuts for the desktop switching might be confusing for users:

• Meta+Alt+Axis shortcuts scrolls desktops with combination of keyboard and mouse/touchpad (Commit #a2bf5900 ("virtualdesktops: use Meta key for axis shortcuts")
• Meta+Ctrl+ArrowKeys directionally switches desktop with keyboard
• Ctrl+F{1-4} toggles desktop by its sequential number

I believe all of these actions should be unified somehow.

One of the ways is to keep Windows-like Meta+Ctrl+Arrowkeys keybindings and change Meta+Alt+axis to Meta+Ctrl+axis (MR 4618).

However, Meta+Ctrl+axis is already used for the "Zoom" effect. It works nicely and pretty discoverable (due to the common usage of Ctrl in browsers and KDE applications).

Therefore, for all desktop switching actions I suggest using combinations based on Meta+Alt (instead of Meta+Ctrl). Such change should make Meta+Alt+axis shortcuts more intuitive and discoverable.

Additional notes:

1. Meta+Alt+ArrowKeys are currently bound to "Switch to Window Above/Belove/Left/Right" actions, and hence they conflict with the proposed shortcuts for "Switch One Desktop Up/Down/Left/Right". I presume that "Switch to Window Above/Belove/Left/Right" actions are not widely used so we can unbind them by default.
2. Ctrl+F{1-4} shortcuts are currently used for the selection of the virtual desktop by its sequential number, I propose rebinding them to Meta+Alt+{1-4}. This should improve consistency and provide a better experience for laptop users with unlocked Function keys.