If asset is referenced by another presentation in the project, it should be indicated in project panel somehow

Currently only assets that are directly referenced by current presentation are highlighted. However, it would be useful to know which assets are referenced from other presentations in the same project, too. For example, deleting assets that are referenced from any presentation should be blocked.

The highlight color for assets referenced from other presentations should be different than the current highlight. Needs UX input.

When resolving the references, the results should be cached, as opening and parsing xml files is a pretty expensive operation for determining highlight. This caching could also be useful for many of PresentationFile and ProjectFile functions, which load and parse .uia and .uip files.