Allow items to be of any class instead of strings.
Item serialization and deserialization methods are in a separate class
because they are also used in a static context.
This new class will help write more logs. The LogPrinter is no longer
created everytime the keyboard is opened.
An error log is added if failing to load the custom extra keys.
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.
