[balooctl] Clarify and extend status output

Summary:
The current index file count does not really tell what the numbers
are referring to - the second number is the count of files currently in
the index, while the first number is the number of files which have either
finished content indexing, are not ellegible for content indexing, or
failed during content indexing.

Provide separate messages for count of files in the index, files waiting
to be processed by the content indexer, and files which failed in the
content indexer.

./bin/balooctl status

Old:

Baloo File Indexer is running
Indexer state: Idle
Indexed 4560 / 4561 files

New:

Baloo File Indexer is running
Indexer state: Idle
Total files indexed: 4561
Files waiting for content indexing: 1
Files failed to index: 2

Test Plan: run balooctl status

Reviewers: Baloo, ngraham, astippich

Reviewed By: Baloo, ngraham, astippich

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21538