Allow recent clips to be pinned. Pinned clips are put a different place
and are persistent.
The pane is split in two columns, the clipboard history and the pinned
clips.
Pinned clips are stored in a new preference file.
Improved pinning layout
clipboard: Remove history entry after pinning
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.
