Disable while fullscreen app is focused

[dyskos]

Fixes Issue #27

Disables hot corner while a focused app is fullscreen. I have verified that it works in the following cases:

• Exclusive fullscreen games
• Borderless fullscreen games
• Fullscreen apps entered with F11/Alt+Enter (such as Firefox)
• Fullscreen videos (YouTube)
• LibreOffice Impress while in presentation

At only 10 additional lines of code, I believe this will make the program more useful for the majority of users, while maintaining the simplicity of the program. This also more accurately replicates Gnome hot corner functionality.

Anyone that does not desire this functionality need only remove these additional lines before compilation.