migrate and adapt indicator box component from workspace navigator

[jjelschen]

This doesn't do anything yet, its just the indicator box component and dependencies migrated and adapted over to this module.

I did make one significant change in that the markers are now stored with each node directly, as opposed to keeping them in a central place (was: the Workspace class), in a map with path strings as keys to look them up. While this has cleared up the code that makes up this PR, it will mean that the way marker values are being set needs to change.

One option would be to traverse the whole tree to find particular nodes to manipulate marker states. Alternatively, we could signal marker changes via message bus events. If both methods turn out to be too expensive, we would need a lookup map, again, which, this time around, I would propose to design as a "tree index", i.e. a map from node id to node object. Downside of that is the additional bookkeeping required to keep the tree structure and its index in sync…

[jjelschen]

Had to turn off source maps for testing, as we have done in other projects, as well, to workaround the Angular CLI bug. However, that bug usually only caused a wrong (generic) error message in failing tests. But in this case, two unit tests failed with the message indicative of said bug, but actually ran green with sourcemaps disabled. So I'm not sure what to make of that…