Implement #190 - pause menu, fix segfault - pressing escape key during netplay segfaults, fix bug - the 'done' flag wasn't cleared between rounds

A static tick callback was added to scene for code that requires to be unaffected by the pause status.
The normal scene tick function will not be executed while the menu is visible.
Merge branch 'master' of https://github.com/omf2097/openomf