add borderless window support

[sphair]

Adds a third window mode - borderless. Borderless windowed mode runs a maximized window without standard title bars or window borders, making it fill the entire screen while acting like a normal desktop window.

[btsouth]

Thanks for contributing borderless mode. It remains a useful distinct option, but this branch currently conflicts with master after the v18 Settings and display/window changes.

Please rebase onto current master so we can review it against the shipped window lifecycle. Before merging, we need passing checks plus Windows validation of mode selection, fullscreen toggling, focus/Windows-key handling, saved placement, secondary display windows and mixed-DPI behavior. The PR currently has no reported checks or review evidence.

We are focusing v1 on the reliability and acceptance gates in #77; borderless mode is optional for that milestone rather than a release blocker.

[sphair]

I will update the branch. With checks, do you mean manual tests I have performed to verify the changes ?