Add EventMouseLeave

[tonsky]

This event should fire when mouse goes outside the window boundary

• Windows
• X11
• macOS

See #227 and HumbleUI/HumbleUI#27 for context

[blnote]

I can try to help with the windows part of the issue, just got it compiled.

[blnote]

Ok, I implemented it for windows and could push a branch for pull request if you permit

[jaketothepast]

I'll take this for Mac and X11