Improvements/fixes to KRunner KCM and krunner-plugininstaller

• krunner-plugininstaller: Improve messages, ensure help can be shown
• krunner-plugininstaller: Fix warning about QDialog already having a layout
• Add krunner_systemsettings as default favorite
  This is also reasonable to have since settings are next to apps the most imporant search features
• kcms/runners/plasmasearch: Adapt text for favorite heading
• kcms/runners: Fix reloading of model when entry is installed