[PVR][input] allow custom keymaps also for PVR radio

this is an alternative approach for #5254 by simplifying and unifying how fallback windows are handled when resolving keymaps and adding a custom fallback strategy for PVR radio on top of that.

I tested the unification/simplification changes for keyboard keymaps (video_menu, fullscreen_info) and they work just like before (as expected), so same should be the case for touch devices which was just a copy of the cApp::onKey() code.