desklets: make desklets lockable

Add ability to lock desklets through the context menu. Right now the
lock state is applied to all desklets and stored in gsettings under the
key `desklets-locked'.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
desklets: close context menu before DND operatios

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>