systemtray: Check if an icon can be located when appending symbolic

David Redondorequested to merge
work/davidre/symbolic-check into master

An app icon directory can either contain icon theme folder structure or be a directory containing files to be used as icons. In the latter case (extraSeachPath in KIconLoader speak) there is no fallback done as there's no icon theme to do fallback in. So check if an icon can be loaded when "symbolic" is appended before doing so. BUG:495754

Merge request reports