[dolphin/search] Add method isIndexEnabled()

Summary: Extracts the logic for checking if the current path is indexed to its own method.

Test Plan: No behavior changes

Reviewers: elvisangelaccio, ngraham, meven

Reviewed By: ngraham, meven

Subscribers: meven, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D24478