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.
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.
The script `check_layout.py` checks some properties about layouts.
No check is an error.
The result of running this script on every layouts is stored in the file
`check_layout.output`, which is useful to track changes.
Add make rules to run this script as well as `sync_translations`.
