Add button to invert Source and Target of an Edge

**Is your feature request related to a problem? Please describe.**
Can not change the direction of the arrow of an Edge.

**Describe the solution you'd like**
Add a button to invert Source and Target

**Describe alternatives you've considered**
Delete and re-create a new edge

**Additional context**
None

No file or test is named in the issue. Start by locating the Edge editing controls and the representation of its Source and Target; done means a button lets users invert them without deleting and recreating the edge, with the updated arrow direction visible.

Written by the indexing model from the issue text.