Add executables path, clean up preferences some

This changeset adds a new game preferences variable for an executables path. This is for games that might have their executable or their BSP compiler somewhere outside of `enginepath` (e.g. `/usr/local/bin`, `/Applications/Game.app/Contents/MacOS`, etc). This variable is entirely optional. It is set in the _Configure a game_ dialog, which has received some aesthetic attention as well. The variable, if set, is available to project templates as `$TEMPLATEexecpath` and should be followed with a trailing slash to be safe.

I also tried to clean up the `CGameInstall::Run` method some, because I was in there already. There's less copy-paste code in it now.