Make ViewController open

[tomaszpieczykolan]

This PR makes the ViewController class open, so that it can be overriden.

[tomaszpieczykolan]

This is work in progress. Before submitting for review, I would like to audit the whole ViewController class and write some more unit tests for it.

[compnerd]

@tomaszpieczykolan would you mind rebasing the change? I think that the new features that I've enabled will be nice to take advantage of.

[codecov[bot]]

Codecov Report

Merging #544 (6b9c15d) into main (fda786e) will increase coverage by 1.19%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
+ Coverage   13.79%   14.98%   +1.19%     
==========================================
  Files         111      111              
  Lines        4443     4443              
==========================================
+ Hits          613      666      +53     
+ Misses       3830     3777      -53     

Impacted Files	Coverage Δ
...s/SwiftWin32/View Controllers/ViewController.swift	46.75% <ø> (+46.75%)	:arrow_up:
...n32/Views and Controls/DirectionalEdgeInsets.swift	43.75% <0.00%> (+12.50%)	:arrow_up:
...in32/App and Environment/ApplicationDelegate.swift	17.54% <0.00%> (+17.54%)	:arrow_up:
...ces/SwiftWin32/Views and Controls/EdgeInsets.swift	33.33% <0.00%> (+33.33%)	:arrow_up:

[egorzhdan]

The ability to subclass ViewController & override buildMenu(with: MenuBuilder) is really helpful for menus, it would be great to merge this PR if there is no concern left.

[compnerd]

I that that when the class is being marked open, we need to ensure that the members are properly marked as open as well, which is not the case yet. Furthermore, @tomaszpieczykolan has this marked as a draft still. I definitely am waiting on this change as I have some changes sitting around that require this to be open.