Add helper function to apply arbitrary GUI tags (such as font and color).
This avoids the caller having to duplicate the GUI tag format over and over again (equal to rP20585),
detects syntax errors at compile time and
applies the separation-of-concerns pattern (callers only have to specify the tag value agnosticly of the tag format).
Differential Revision: https://code.wildfiregames.com/D1167
Patch By: fpre / ffffffff
Comments By: bb
Refs rP16262