Add missing category "text" to file search results

The baloo runner plugin was missing the cateogry "Text" in the matches, which caused many files to be inaccessible from KRunner even though they are indexed and relevant. This change fixes the missing results and correctly finds me all relevant files.

See https://www.reddit.com/r/kde/comments/tlt6bv/why_baloo_does_not_provide_all_its_results_to/.