feat: add minimize button and windowed mode for easier window management

[riammadber]

Problem

Teachers cannot minimize OpenBoard during live classes because
the app runs in borderless fullscreen mode with no minimize button.

Solution

Added a minimize button to the toolbar that calls showMinimized().
Also added a windowed/fullscreen toggle in Preferences → Display.

Testing

Tested on Windows 11, OpenBoard v1.7.7

[Vekhir]

Teachers cannot minimize OpenBoard during live classes because the app runs in borderless fullscreen mode with no minimize button.

Are you aware of the Desktop Mode? With it, you can fully access the underlying system without leaving OpenBoard. Maybe that's a better solution to your problem.

[sebojolais]

Yes, this feature is already there.
Have a look at this ticket.
It is better to open a ticket to discuss the topic before providing a PR.