A new option allow to choose a secondary layout, the switching key is
placed on the top edge of the space bar.
The "Programming layout" option was basically doing that but it was
possible to choose from a few layouts only. It is improved and renamed.
The 'LayoutListPreference' allows setting the string for the first entry
but otherwise share the rest of the array.
Add nice icons from materialdesignicons.
The key enable caps lock immediately. It does nothing if caps lock is
already enabled.
It is not present on the keyboard by default but a place is defined on
every layout, top-right of the shift key. It can be enabled in the
settings.
The icon is from materialdesignicons.com.
