Improve key mapping to fix menu shortcuts in more cases

[rburchell]

The mapping of QKeySequence to gtk accel requires us to map Qt::Key to GDK_KEY_*. The reverse of the awful key mapping we did before.

Some keys have already been mapped, but there's a lot more to do. See qt_convertToGdkKeyval.

[rburchell]

[this is a mass update]

This is polite notice that gtkplatform is effectively end of life. Due to this, I've decided to archive the project.

I (and I think, the other contributors) have not been using it in quite some time, and I don't see that situation changing any time soon. In addition, none of us really have the time that it takes to do what it takes to keep the lights on, and this situation is going to get a lot harder soon with the impending arrival of Qt 6 and gtk 4 on the horizon.

Should anyone wish to take it over, you have my blessing to do so :)