Move data/ to MacOS/Resources/ in Mac bundle.

This fixes the various issues with needing to
code-sign data files.  And simplifies the process
of building and preparing to code sign considerably.
Implement FileOpen event

This allows Quackle to respond to system file open
events.  So, if .gcg is associated with Quackle, then
double-clicking a .gcg file will now make it open in
Quackle (whether it's running or not).  And drag and
drop, etc.

This definitely fixes issues trying to open files with
double-click on Mac.  Windows doesn't have the
.gcg association set up yet, so code is untested
there, but it should be platform independent.