Add GenericWindow

[saihaze]

GenericWindow is added to abstract over both Wayland and X11 windows.
Some comments are copied from Window.
Anvil hasn't been modified yet. It's WIP.

[codecov-commenter]

Codecov Report

Attention: 271 lines in your changes are missing coverage. Please review.

Comparison is base (859ed44) 21.74% compared to head (11ea062) 21.91%.

Files	Patch %	Lines
src/desktop/wayland/generic_window.rs	38.52%	217 Missing ⚠️
anvil/src/shell/element.rs	37.50%	40 Missing ⚠️
anvil/src/shell/ssd.rs	0.00%	7 Missing ⚠️
anvil/src/shell/grabs.rs	0.00%	4 Missing ⚠️
anvil/src/input_handler.rs	0.00%	2 Missing ⚠️
anvil/src/focus.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1233      +/-   ##
==========================================
+ Coverage   21.74%   21.91%   +0.17%     
==========================================
  Files         155      156       +1     
  Lines       24451    24626     +175     
==========================================
+ Hits         5316     5398      +82     
- Misses      19135    19228      +93     

Flag	Coverage Δ
wlcs-buffer	18.85% <27.16%> (+0.08%)	⬆️
wlcs-core	18.52% <28.99%> (+0.09%)	⬆️
wlcs-output	7.60% <0.00%> (-0.06%)	⬇️
wlcs-pointer-input	20.69% <36.30%> (+0.12%)	⬆️

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[saihaze]

Related to #1202

[Drakulix]

Looks quite useful already!