resultset: fix agent escape string

If agent name had ' in it, it was not escaped properly. This replaces ' with \' so the string doesn't escape early.

For example (agent = 'Baldur's Gate Dark Alliance') broke the query.

Testing: Make a desktop file in ~/.local/applications with app name that has ' in it and run plasmashell with QT_LOGGING_RULES="*.debug=true;qt.*=false;org.kde.plasma.pulseaudio=false" plasmashell --replace

(cherry picked from commit 5f7ec2dc)