Reset prop to NULL before re-testing it. Also conditionalize .game  generation.
Add new game preferences variable for executables path. This allows games whose binaries live outside of the engine / data path to be resolved in .proj templates and the like. The executables path can be set via the game preferences dialog.

This changeset also cleans up the game preferences dialog and especially the game configuration dialog by adding frames and fixing padding where necessary.

Lastly, this changeset simplifies the prefix / prefix_win32 code to work like the rest of the conditionalized template attributes (via macro).

Oh, I almost forgot, the CGameInstall::Run() method is a bit less ugly now too. It's still very ugly.
Merge branch 'master' of https://github.com/TTimo/GtkRadiant
Merge pull request #198 from jdolan/master

Add executables path, clean up preferences some