avatar-chooser: make the file chooser dialog modal

https://bugzilla.gnome.org/show_bug.cgi?id=706881
build: add --disable-settings to avoid installing the GSettings schema

This is needed to avoid multiple components shipping the same schema.
It's just a temporary workaround that won't be needed when
https://bugzilla.gnome.org/show_bug.cgi?id=706317 will be fixed.

https://bugzilla.gnome.org/show_bug.cgi?id=706801