appstream: Don't assume the first subcategory is always called "all"

Instead of always leaving out the first subcategory, be a bit more
careful and do an ID comparison to skip the correct category we want to
skip.

This fixes Addon::Font to correctly show up again in the category view
(regression from commit d3648252).

Fixes: #133 (comment 509825)
1 job for <span class="ref-name">wip/kalev/fonts-missing</span> in 22 minutes and 54 seconds
Status Job ID Name Coverage
  Build
passed #298630
build-gnome-software

00:22:54