Open folders in the system's file browser on double click

[msteghofer]

Double click on any folder of the CSnake GUI should open that folder in the system's file browser.

This feature is partially implemented, but works only on Windows and only for the root folder.

In theory the third party source and build folders also had this feature, but lost it when switching to the new table layout - the remainders of this implementation (the functions OnThirdPartyFoldersDClick and OnThirdPartyBuildFoldersDClick and their registration in InitFrame) have to be removed or reactivated, too.