appimage: Simplify deployment of Okular and its plugins
* src/appimage/AppRun: Export QT_PLUGIN_PATH. * src/appimage/build-appimage.sh: Copy the KDE plugins to AppDir/usr/lib/plugins instead of AppDir/usr/plugins. Remove manual copying of Okular's plugins and dependencies. Make patchelf verbose. Loop over all files in AppDir/usr/lib/plugins to patch their rpaths.
This is based on similar changes made with 0da1d1de for the Qt6-based
AppImage.
- GnuPG-bug-id: T7525