* docker/appimage/build-appimage.sh: Use paths in installation directory
instead of AppDir where necessary. Also remove an already existing
AppRun.
* src/appimage/appimage.mk.in (idir): Change to directory outside of
AppDir.
(rsyncfilterdir): New variable.
($(stampdir)/stamp-directories): Create $(appdir)/usr.
(SETVARS): Add pkgrsyncfilter variable.
($(stampdir)/stamp-$(1)-06-copy-to-appdir): New targets.
($(stampdir)/stamp-final-$(1)): Depend on new target.
(clean-appimage): Remove build directory and installation directory
instead of (unremovable in Docker container) root directory.
src/appimage/rsync-filters/*.rsync-filter: New.