Work in progress: It's not yet possible to paste from the pane.
The pane can be switched to and from and displays the strings recently
added to the clipboard.
ClipboardHistoryService listens for change to the system clipboard and
keep the history in memory.
This data is not persisted to the storage.
The maximum size limits the amount of user data stored in memory but
also gives a sense to the user that the history is not persisted and can
be forgotten as soon as the app stops.
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.
