[Appearance] Interface density

[git-f0x]

This adds the 3 interface density options from the designs: Comfortable, Compact and Spacious.
Should be tested alongside pop-os/libcosmic#603.

[TheESN]

Can you show screenshots with examples of these settings please?

[git-f0x]

The changes aren't as extensive as they should be, since a lot of things aren't yet using the variables, and this should possibly change other things as well, like the headerbar height (edit: it seems the header bars already have some density variables in the code, so I might be able to do something) or NavBar width (though not sure if the NavBar will be manually resizable in the future).
But here's a comparison of Files with the current values (Comfortable), and Compact (but as mentioned, the exact values are currently arbitrary):

[git-f0x]

This is pretty much ready, as far as the Settings side is concerned. Could possibly be merged with the sections.insert(interface_density()), line being commented out, or wait until everything in libcosmic and apps is polished.
Though merging as is would enable easier testing of what needs tweaks for all COSMIC components, but would introduce something that is a work in progress to users (maybe a disclaimer in the section title that says that it's a work in progress could be used).

[mmstick]

I think interface density should be part of Experimental Settings for now.

[git-f0x]

I'll try to fix some App Library issues when Spacious, and then I'll try moving it to the Context drawer (above or below icon themes?).

[mmstick]

I'll do some testing. Perhaps we don't need it in experimental settings if it's already working well.

[git-f0x]

It looks slightly ugly sub-optimal in some cases, but when apps update their libcosmic version, there shouldn't be many things that are cut off or broken. I haven't found anything like that, other than in the App Library, and App Tray (my PR in applets fixes this).

[mmstick]

It seems good enough to me.

[mmstick]

Thanks!

[git-f0x]

I'll work on fixing the App Library, so there isn't missing text on group buttons when Spacious.