This commit adds two signals:
* connection_drag_begun, which is emitted when a connection is started
to be created by the user and
* `connection_drag_ended`, which is emitted when no longer a connection
is created.
Additionally `force_connection_drag_end()` adds the possibility to end
the connection dragging. If called from user code, no other connection
request signals are invoked. This is useful to add `GraphNode`s via
shortcuts while the user is dragging a connection to directly connect
the newly added node.
0505b08a56