ClipboardsManager: add a way to sync two clipboards

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
keybinder: add a keybinding to sync the primary to clipboard

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gpasted: use the new keybinding

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>