Introduce a DesktopWorkspaceSwitcher

[leolost2605]

Takes over the responsibility for animating the workspace switch from the window manager. It also prepares for switching over multiple workspaces at once with one gesture.

This has multiple improvements:

• Allow switching over multiple workspaces with one gesture (infrastructure in place, only requires new touchpad backend)
• Allow interrupting a gesture (requires changes from MultitaskingView: Allow workspace switch animation to be interrupted #2161)
• Allow interrupting keyboard shortcut animation
• Allow to spam keyboard shortcuts (especially switch to next/previous ws) to quickly switch over multiple workspaces
• Animate over all workspaces that are skipped (e.g. when triggering move to last ws shortcut)

Known issues:

• When interrupting a gesture the bounds might not be set correctly (Requires changes from MultitaskingView: Allow workspace switch animation to be interrupted #2161)
• When spamming shortcuts that would trigger nudges the nudge "stack" and we get some weird behavior (Also requires changes from MultitaskingView: Allow workspace switch animation to be interrupted #2161)
• The dock will always show (without animation as well) even if it is hidden (requires changes from Introduce a shell group #2195)
• Notifications aren't handled (will be automatically done by Introduce a shell group #2195)

[leolost2605]

This is ready for testing and review now.
However it requires changes from #2195 and #2161 to fix some rare cases of unwanted behavior (e.g. the dock will always show up during switching, and one can stack the nudge infinitely) therefore

THIS IS NOT READY FOR MERGING YET (but since it's quite a big change that's pretty much in its final form testing and code review is already welcome :))