Do not include unneeded Qt translations in the AppImage

* docker/appimage/build-appimage.sh: Do not copy unneeded translations.
Remove existing translations.
* patches-appimage/ki18n/
0001-Load-Qt-translations-even-if-some-catalogs-are-missi.patch: New.

Add upstream patch that makes loading of Qt translations more robust.
This allows us to leave the Qt translations of qtscript, qtmultimedia
and qtxmlpatterns out of the AppImage.

• GnuPG-bug-id: T5592