Implement hand written marshalling

The scripts are now encapsulated in the distinctive struct
'ScriptArchive', which can read from and write to QIODevices
in either Ruby 1.8 (XP) or Ruby 1.9 (VX Ace) format.

This allows us to be independent of the linked Ruby version.
Adapt the UI code to the new marshalling functions
Merge file dialog script filter into one
Fix crash when opening script file

'RGSS_MainWindow::setCurrentIndex()' did not respond very well
to idx == -1, so I fixed that. Also took care of some corner
cases that arise in between combinations of Open and Close.