Don't use relative sizes for computing the category sidebar width

This can lead to the window unexpectedly resizing, depending on the
content. Instead, just hardcode the preferred width.

https://bugzilla.gnome.org/show_bug.cgi?id=734783
Do not use the GTK icon cache when loading AppStream icons

We're doing this in a thread, and the icon cache is not thread safe.
Never schedule more than one updates reload