Option to add extra/exotic keyboard layouts

I like the new keyboard layout settings in elentaryOS 5.0.
It's a huge improvement compared to 0.4.

The layout settings only show the layouts, which are stored in `usr/share/X11/xkb/rules/base.xml`. (resp. `evdev.xml`)
There are more (extra/exotic) layouts in `base.extras.xml` (resp. `evdev.extras.xml`), which are not listed.

E.g. in KDE Plasma, those are listed by default as well.
In other desktop environments (GNOME, Cinnamon, MATE) it is possible to show those extra layouts with one of the following keys:
```
gsettings set org.gnome.desktop.input-sources show-all-sources true
gsettings set org.gnome.libgnomekbd.desktop load-extra-items true
gsettings set org.mate.peripherals-keyboard-xkb.general load-extra-items true
```

It would be nice, if either those items where shown by default in elementaryOS, or having a key/option to enable those extra layouts.