Fix several issues raised in #7149:
* When search scope is a single file, skip exclusion filter entirely
* 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 to centralize the filtering code more, and
simplify _doSearchInOneFile() & its call sites.
Code review: restyle filter editor file count and make UI element widths
cleaner. Fix some stale docs and other nits.
Merge pull request #7400 from adobe/pflynn/exclusions-improvements

File exclusions usability improvements (bug #7149)