support sxmo keyboard in installer

After having used the Sxmo keyboard for a bit, I like it! It would be nice if the installer would use it too if Sxmo is going to be installed. For other UIs it probably does not make so much sense, because their keyboards look more similar to the keyboard we already have.

postmarketos-ondev uses X11 (for best compat with all devices postmarketOS has been ported to), so this would be feasible if we add an option in the mobile module of calamares-extensions to not use the qtvirtualkeyboard, and then add code in postmarketos-ondev to set that option and use the sxmo keyboard if sxmo is going to be installed.