Remove edges from toolbar, not by dragging

[bergie]

Currently it is too easy to accidentally start dragging the plug of an existing connection and remove it when intending to make a new connection. This is an issue with mouse, but even more so with touch (see #53).

Instead, edge removal should happen just like node removal happens, by providing a delete (disconnect?) button in the contextbar when an edge (or set of edges) is selected.

For convenience this should also be possible by clicking an edge and then pressing the delete key on keyboard.