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.
Allows to add more keys to the keyboard from a predefined list.
The implementation doesn't use MultiSelectListPreference because it
doesn't seem possible to change the item layout to properly show the
rendered symbols.
