Miscellaneous fixes and small refactors

Synopsis:

• Send linux-dmabuf events on bind only to this bind. This error is easy to make and difficult to spot. It might be better to rename the send function for binds to send_to or the send function that broadcasts the event to all to send_all or broadcast.
• A data source might be down already when the offer is accepted. Check for that.
• Clean up our Server internals after the Nucleus class was introduced: the Capsule class can be replaced with a simpler BasicNucleus class.
• Handle drag target going away.

Closes kwinft#69 (closed)