Use a custom AppRun script

* docker/appimage/build-appimage.sh: Remove existing wrapped AppRun.
Pass custom AppRun to linuxdeploy.
* src/appimage/AppRun: New.

The custom AppRun adds ${APPDIR}/usr/bin to the PATH, so that Kleopatra
uses the GnuPG executables included in the AppImage. Moreover, it
shuts down GnuPG daemons that have been started from the AppImage
because the AppImage will be unmounted once AppRun exits.

• GnuPG-bug-id: T5592