Add a very basic virtual desktop module

[andydotxyz]

This is the bare bones. It is functionally complete but slow :(.

Mostly the slowness is due to unmapping/mapping all the windows.
I don't know how else to do it just now - once we have a built-in compositor we can do it super fast by just not drawing...

Fixes #186

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.
• Public APIs match existing style.

[coveralls]

Pull Request Test Coverage Report for Build 3934180783

• 12 of 66 (18.18%) changed or added relevant lines in 7 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 29.248%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/ui/switcher.go	4	5	80.0%
internal/ui/bar.go	0	4	0.0%
internal/ui/desk.go	7	16	43.75%
wm/border.go	0	10	0.0%
internal/x11/wm/desk.go	0	11	0.0%
internal/x11/wm/events.go	0	19	0.0%

Files with Coverage Reduction	New Missed Lines	%
internal/x11/wm/desk.go	1	1.22%

Totals
Change from base Build 3934176036:	-0.2%
Covered Lines:	1544
Relevant Lines:	5279

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💛 - Coveralls

[andydotxyz]

No negative reviews, PR defaulting to accepted - it has been played with in the community