Fix shortcut saving on KDE, and use the system dialog for Frameworks

While re-enabling the system dialog for shortcut editing for KF5,
I finally figured out why shortcuts where never saved when using KDE;
a long standing bug we've had.
Turns out QDialog::exec() does not actually save things. All this time
I was looking into completely different directions; but it never
occured to me that I need to explicitly save after the dialog is
accepted (our own dialog does this automatically). This is not a
complaint; it was being stupid on my end :)

Fixes #1057.