implement four finger gestures to hide/show the cosmic app library and workspace overview

[FreddyFunk]

Adds new four-finger trackpad gestures to quickly open and close the workspace overview and app library.

Four-finger gestures

• Swipe left/right to switch workspaces (when using horizontal layout)
• Swipe up/down to switch workspaces (when using vertical layout)
• NEW: Swipe up to show workspace overview and swipe down to show app library (when using horizontal layout)
• NEW: Swipe left to show workspace overview and swipe right to show app library (when using vertical layout)
• CHANGED: Switch workspace gestures now work even when workspace overview is already open (previously blocked)

How it works

The gestures adapt to your workspace layout: in horizontal layout, left/right swipes switch workspaces while up/down toggles overview/app library. In vertical layout, it's the opposite.

Swiping in one direction opens the workspace overview (or closes the app library if open). Swiping in the opposite direction opens the app library (or closes the overview if open).

Natural scrolling settings are respected. If you have natural scrolling enabled, the gesture directions are reversed to feel more intuitive.

Fixes pop-os/cosmic-epoch#1367
Fixes pop-os/cosmic-epoch#636
Fixes #801

And kind of adresses:
#315

[Drakulix]

Thanks a bunch for all this work, but I would rather rework and merge #396 at some point to allow people to customize the gesture before adding more of them. (As disabling some of these features will inevitably come up.)

CHANGED: Switch workspace gestures now work even when workspace overview is already open (previously blocked)

Also this is intentional and something to be handled by the workspace-overview.

[leviport]

For UX: I pushed this to a staging branch called testing-cosmic-comp-pr1799 so CI will build it and it can be tested more easily from a popdev source.

[FreddyFunk]

Thanks a bunch for all this work, but I would rather rework and merge #396 at some point to allow people to customize the gesture before adding more of them. (As disabling some of these features will inevitably come up.)

I would be open to implement a proper settings configuration soon. I could propose some possible settings and UI for those settings, but in case anyone from the UX/UI team would be open to provide some kind of pre-approved design, I'd take that to prevent unnecessary work.
Nonetheless, I would be happy if this PR could be merged before this happens so users with trackpads can daily drive cosmic with a bit more comfort in the meantime.

CHANGED: Switch workspace gestures now work even when workspace overview is already open (previously blocked)

Also this is intentional and something to be handled by the workspace-overview.

All I can say is that this new behavior "feels right" for me from an UX experience.
I can move the gesture handler to a different module, if it makes more sense from an architecture perspective. I'd honestly appreciate this kind of feedback, since I'm quite new to this whole project :)