Draw version in lower left corner, take 2

 * Version drawing and FPS drawing now done in the same
   function, toggled by F3.
Add a screenshot manager, and let it save the screenshots with a nice name
Merge pull request #62 from franciscod/screenshot

screenshot: refactor with numbering and timestamping
Merge branch 'draw_version' of https://github.com/oldlaptop/openage into oldlaptop-draw_version

Conflicts:
	copying.md
	cpp/engine.h
engine: removed 'version' text from version overlay