Fix mouse events interaction with ALT/F10 on Windows.

[xStrom]

This issue was reported by @Azorlogh in zulip.

When pressing (just once, not holding) the ALT key or F10 key, the window stops receiving mouse events. This can be reproduced with e.g. the timer example.

The reason is that ALT/F10 are system hotkeys that focus the menu. However when there is no menu these hotkeys aren't useful. This PR fixes the issue by suppressing the system handling of these keys when the window has no menu.

I've made a table to help test the changes in this PR.

.	The multiwin example	The timer example
master	Pressing ALT focuses the menu and hovering over the buttons doesn't change their appearance.	Pressing ALT stops mouse events so the box won't turn red due to mouse movement.
This PR	Pressing ALT focuses the menu and hovering over the buttons doesn't change their appearance.	Pressing ALT does nothing.

[cmyr]

looks like a reasonable fix!