fix: improve the way frameless windows are handled on Windows

[brenca]

Description of Change

Fixes #15816.

Most of the ideas implemented here come from this post, which is mentioned in several comments. Particularly the idea to use insets on the RootView comes from this part:

When the window is maximized, Windows will clip out the border on the app’s behalf if a clipping region hasn’t been applied. In this scenario there’s no reason to have done that, but the thing to watch out for is if glass isn’t extended as deeply into the borders as Windows would have (SM_CXFRAME (+ SM_CXPADDEDBORDER on Vista)) then a maximized window will clip the edges of your client region. The simplest way to work around this is to just place a Border around the window’s content that has a thickness of clipping region, and make its Visibility collapsed when not maximized.

I also took inspiration from this repo, but decided to not mess with undocumented events.

I tested these changes when I put this PR together on Windows 10, 8.1, and 7.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes:
Improved the way frameless windows are handled on Windows.

[zcbenz]

CI failed on Linux with:

../../electron/atom/browser/native_window_views.cc:1103:12: error: cannot initialize return object of type 'gfx::AcceleratedWidget' (aka 'unsigned long') with an rvalue of type 'nullptr_t'
    return nullptr;

[release-clerk]

Release Notes Persisted

Improved the way frameless windows are handled on Windows.