runners/baloo: Add missing category "text" to file search results

The baloo runner plugin was missing the category "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 all
relevant files. 

(cherry picked from commit bd397f89)
2 jobs for Plasma/5.24 in 8 minutes and 38 seconds (queued for 2 minutes and 56 seconds)