appimage: Improve build of Breeze icons

* docker/appimage/Dockerfile: Add gtk-update-icon-cache and
python36-lxml.
* src/appimage/Makefile.am (appimage_pkg_breeze_icons_configure): New.

This change

• disables the creation of the unused huge icon resource files which

bundle all icons in a single file,

• enables the creation of the icon cache (with gtk-update-icon-cache),

and

• enables the generation of 24x24 icons (using Python 3 with lxml).

• GnuPG-bug-id: T5592