[win32] new default install folder for GRE

[Sean Su]

Was there a specific reason for wanting/needing GRE to be installed to [program files]\mozilla.org\GRE by default for Windows? I'd like to propose a more central, shared location than the one currently used:
[common files]\mozilla.org\GRE

where [common files] is the recommended folder to place shared files under windows, and where it's location is normally:
[program files]\common files

There's already a bug filed for moving GRE to the [common files] folder: http://bugzilla.mozilla.org/show_bug.cgi?id=190714
If there's any objection, please speak up soon. I'd like to get this in for 1.3b.