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.
Incompatible APIs were used in the custom layouts and the extra keys
options.
Add @TargetApi annotations to help catch similar issues in the future
with the help of 'gradle lint'.
This separates the layout definitions from the special layouts
(bottom_row, greekmath) and other unrelated files (method, settings).
This is also a more intuitive location for layouts and make the resource
directory easier to navigate.
Under the hood, layouts are copied back into
build/generated-resources/xml.
