Allow plugins to add topicmap and workspace commands
Plugin devs need a possibility to mount further workspace commands resp. topicmap commands. A command is basically a Vue component:
Workspace and topicmap commands will be maptype-specific.
That is the dev will specify a maptype (URI) when registering such commands.
When switching the topicmap the Webclient will display only those commands registered for the current maptype then.
Note that e.g. a geomap or a tableview is not meant to reveal arbitrary topics on it. So e.g. the i-command (as shown above) makes no sense on a geomap or tableview (and is not functional at the moment). As a consequence a geomap (or tableview) can't be deleted at the moment.
With the new command facility e.g. the Geomaps developer would register a "delete" topicmap command, and handle it itself.
This is a follow-up of #116, #373 (closed), and a forum posting