Switch dragging and connection modes

Right now, dragging a non-selected node starts the edge drawer. Intuitively, dragging a node should move the node around. We can change this by adding an "edge drawing" mode on the top bar, which needs to be active to draw edges.

Start by running the modeler and locating the node-dragging behavior and the top-bar controls. Trace how dragging a non-selected node currently opens the edge drawer, then determine where an edge-drawing mode can be represented. Done means ordinary dragging moves nodes, while edge creation works only when that mode is active.

Written by the indexing model from the issue text.