fileindexerconfig: Introduce canBeSearched()

Summary:
Prepare for search refinement in dolphin.

The option to use baloo for searching should not only depend on the indexing state of search start folder but also on the indexing state of its descendants.

Example:
  - ~/  : not indexed
  - ~/Videos: indexed
  - ~/EBooks: indexed
When starting search from ~/ the user should be able to use baloo for search.

CCBUG:388656

Test Plan: unit test, upcoming

Reviewers: #frameworks, #dolphin, #baloo, mgallien, ngraham

Reviewed By: #baloo, ngraham

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D10205