The new keys are placed on the corner of the arrow keys. Key
descriptions are added.

They are removed from the Fn layer to avoid showing up twice.

The new PreferredPos class represents where an extra key should be
placed
Currently used to place keys at the same positions they were placed
before.

These keys don't seem to have a purpose, which is confusing.

And add description strings.

The new strings are used in the "Add keys to the keyboard" option.

Key descriptions are shown in the "extra keys" option and can now be
translated.

* 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.

This removes the need for a layout definition and adds a title before
the prefs.
It's a prerequise for the custom keys preference.