File exclusions usability improvements (bug #7149)

Fix several issues raised in #7149:
* When search scope is a single file, skip exclusion filter entirely (and hide it from search UI)
* If exclusions filter results in 0 files to search, show a message to that effect and leave search bar open so user can adjust filter
* When editing a filter, show how many files are still included out of the total number of files in the current search scope

Also cleans up FindInFiles a little bit to centralize the filtering code more, and simplify `_doSearchInOneFile()` & its call sites.