- during installation add to the list of custom keyboards if necessary
- during uninstallation remove from the list of custom keyboards
- if list of custom keyboards has invalid values, remove those before
saving the list
Closes#8598.
After this change uninstalling a keyboard will remove all installed
language/keyboard combinations for that particular keyboard.
Previously we only uninstalled for the first language listed in the
keyboard.
This change refactors and cleans uninstall:
- rename methods to make it clear which ones are private/protected
- extract common code
- improve error reporting
