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.
The Fn+switch_numeric shortcut might be hard to discover or too slow for
heavy users.
A location is allocated for the key so it has a consistent placement.
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.
