Ensure that programmatically closing the main window triggers on_exit handling

[freakboy3742]

Derived from #2244

If you programmatically close a window, and the window is the main window, this doesn't trigger on_exit() handling (which could, in theory, reject the close).

This also means that a MainWindow can now have an on_close handler.

This PR moves the MainWindow class to window.py, in preparation for additional changes related to window handling.

Reviewing note: I've found the Window example app very helpful in manual testing of this; it contains a MainWindow that has an on_exit handler, as well as an option to create child windows, one of which has on_close handling.

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