If the key value can emit text, it should put the text to be emitted in
the text field. It should not put any special symbol for the key label,
and it should instead be a special case in the display_label() method.
Some keys may have no XKB key symbol, but still have the ability to emit
text. Examples include the "‹" (U+2039) and "›" (U+203A) characters. Add
the text property to key parts so they can emit text even with no key
symbol.
Also, for keys that were not modified by the Function key because they
had no XKB key symbol name, modify them.
Keeping the keys pressed can confuse some compositors, and the key codes
may change. Release any keys that are pressed while the keymap is
changing, and press them again afterwards.
