* Add section mark as an extra key in QWERTY-like layouts that don't have it already
* Add dagger (also double dagger) as an extra key in QWERTY-like layouts
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.
The current approach is hard to maintain, for example the last key
"autofill" was not displayed.
This implements a PreferenceGroup that contains the check boxes for
every extra keys without involving listing the preferences in
settings.xml.
A custom layout is used to remove the 'title' text view.
The list of extra keys is moved into the new class.
'ExtraKeyCheckBoxPreference' becomes a nested class.
