Clarify which file to select to open a trace

* e.g. Special file icon for the root of a trace file, grey out sub-folders/files
* Some Trace Compass users are trying to click on the metadata file to open a trace.

Start by locating the trace-file selection and opening entry points in the Theia trace extension, then inspect how metadata files, subfolders, and trace roots are currently presented. Done means users can distinguish the trace root from metadata and nested files, with the intended selectable or disabled states implemented and verified in the relevant UI tests.

Written by the indexing model from the issue text.