Added support for set_full_screen in WinForms.

[proneon267]

Added support for set_full_screen so that both enter_full_screen and exit_full_screen works in WinForms.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[proneon267]

I have changed back to is_full_screen instead of set_full_screen because of test error. But, I still think the argument should be named set_full_screen, since that's the condition that is being passed down from enter_full_screen and exit_full_screen.

Anyways, I have tested it locally. Works fine.

[freakboy3742]

Thanks! I've extended your patch to add an example (which itself pointed out a minor bug on the macOS implementation).

I've also simplified the changenote. The change note generally shouldn't take about implementation specifics - it's a high level release note, so it should be framed in terms of talking about the benefits to the end user, rather detailing a specific API.