training screen: fix key event handling wrt dead keys

Turns the event handling in QML is too limited to properly support dead
keys. At least on X11 dead key handling is part of some input method
and one has support for them in QML. Therefore the event handling part
of the training line has been moved to C++.

While I'm at it I also made sure that the training widget acts more line
standard input widget, so Ctrl+Backspace now deletes the last word.

BUG: 309197
FIXED-IN: 2.0