Heatmap/counts in tree view

[lars-reimann]

Is your feature request related to a problem? Please describe

It's not easily possible to identify elements in the tree view that might lack annotations.

Desired solution

Color the entries in the tree view like a heatmap according to some counter and show this counter. The counter can be one of the following options:

• How often is the element used?
• How many annotations does the element have?

Additional context (optional)

Heatmap: Relevant elements are red, less relevant elements are blue. The exact number should also be shown.
Heatmap should be able to be turned off and you should be able to choose from heatmaps (amount of uses, amount of annotations).

• Relates to Load usage data into editor #494.
• Is similar to API Editor: Additional Filter commands #487.