This is a new section in the extra keys option that allows to enter
arbitrary strings which are then added to the keyboard.
A new string is needed for the title of the section, Android's icons and
strings are used as much as possible to avoid adding more strings.
Keys are stored in the preferences as a JSON array of strings.
Values like 'characterSize' and 'horizontalMargin' can't be fed back into the default value because they are not of the same unit.
To avoid this happening again, change the way the default value is defined for every options.
The 'key_height' dimension was no longer used.
