Add `GraphEdit` drag notifications

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.

0 files changed, 0 insertions, 0 deletions