The "Quick Text Search" is probably the most used STS-only feature, thanks for that.
The Idea Community has this kind of search functionality for quite a long time.

But for larger projects its possible that you have many false positive results, so it would be nice that you could filter the results.

• Filename
• Extension
• Path
• Package
• more (size etc, see the idea dialog)

A suggestion could be a second input field where its possible to reduce the source files with a regular expression when "real" UI extension with lots of fields is too cumbersome.