Remember main window position on next launch

I asked for this feature on this forum thread:

http://forum.spritebuilder.com/t/feature-remember-window-size-position/1023

Since there appeared to be no interest in implementing this, I decided to go ahead and implement it myself quickly :) This functionality could probably be cleaned up a bit to do a bit more but for the time being, this should achieve what I wanted. Hopefully, this can be merged into the code base.