Damn I'm lucky: QKeySequence.toString()'s output can actually contain spaces, for example in "Multimedia Play".

I'm lucky because it hasn't been possible to assign multiple keys to an action so let's just redefine the splitter char to tab.

Don't worry, the Tab key actually produces the text "Tab" so we should be safe now.

This fixes nasty problems when using multimedia keys for global shortcuts.
Also add protection against nonpositive keycodes to keysFromString().