This merges the "Layouts" option with the "Custom layout" option.
A custom layout becomes an item in the "Layouts" list among the other
layouts. It's possible to add several custom layouts.
Selecting the "Custom layout" item in the list opens a second dialog for
entering the layout description.
Layouts are serialized as JSON object and are decoded solely in the
LayoutsPreference class.
The two layout selection options are replaced by a ListGroupPreference
that allow to enter an arbitrary amount of layouts.
The "switch_second" and "switch_second_back" keys are replaced by
"switch_forward" and "switch_backward", which allow to cycle through the
selected layouts in two directions.
Layouts are changed to place these two key on the space bar.
The backward key is not shown if there's only two 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 newer haptic feedback API that is used instead of the vibrator
service since ef03dfe doesn't work for everyone.
The new vibration option allow to choose both the newer API ("system")
and the older API ("strong", "medium", "light").
