gtk auto-mnemonics support

Thanks to Simon Long over at raspberrypi, we now have auto-mnemonics support in LibreOffice under gtk3 and gtk2. So the underlines appear in dialogs when alt is pressed and disappear when released, while menus activated from the keyboard show underlines while those activated from the mouse don't. Gives a more native flavour to the UI. Only in 5.1 for the moment.