fix: keyboard shortcuts for left/right workspaces

[laikq]

Add keyboard shortcuts for left/right workspace switching & window moving.

• SUPER + Ctrl + left/right for workspace switching
• SUPER + Ctrl + Shift + left/right for moving windows between workspaces

Fix #1776

Hopefully useful for some people installing pop-shell on vanilla gnome.

[jacobgkau]

If we did want to apply this to master_noble, we'd want to give some thought to whether a different pair of shortcuts could take on the role of snapping windows to the left/right. Although I agree navigating workspaces is probably more useful than snapping as a keyboard shortcut (especially since snapping happens outside of the tiling workflow), snapping has long been part of the Pop!_Shell regression testing checklist and is thus a feature we intended to support with the Pop!_Shell experience. (The simplest solution would be swapping it to Ctrl-Super-Up/Down since those wouldn't be in use with horizontal workspaces, but there'd be no intuitive answer to whether Up should snap left or right, and vice versa.)