Canvas: add "line direction" option to toggle / reverse arrow direction

Use case or problem

Sometimes when connecting cards, after making an arrow from card A to card B, later I realize it would be better to have the arrow going from B to A instead.

Proposed solution

There is already a “Line direction” menu that appears in the context menu when an arrow is clicked, and it gives options for the line to be nondirectional, unidirectional, and bidirectional. But the “unidirectional” option is locked in to the arrow direction that was initially chosen (e.g. from card A to B). Adding one more option to this menu to toggle / reverse the current unidirectional arrow direction would do the trick.