Refactor of TopBarCreate

TopBarCreate now comes with property-value boxes for all but the most common properties, fixing #71.

Link Creation can be cancelled at any point before the target node is chosen, though start and target are still chosen at the end of link creation.

The LinkHover and Expander plugins are no longer allowed to overwrite toplink-instructions while link creation is in progress.

I've kept the write-in box for node colour that has been added recently, though that doesn't work quite correctly with some of the other functionality (e.g. the background colour of node detail view, or the colour picker in NodeEdit). It might be best to replace that with the actual colour picker plugin.