Add as_app_add_veto()

This is simply moving asb_add_add_veto() into the base-class and in turn makes
it stable API.
Add as_utils_find_icon_filename()

This splits out the functionality from the desktop builder plugin.
Add AS_APP_PARSE_FLAG_ALLOW_VETO

This allows us to get the reasons the application was vetoed.
Add AS_STORE_LOAD_FLAG_ALLOW_VETO

This allows us to add applications to the AsStore that would normally not be
added because they have been vetoed.
Add check-root to appstream-util

This allows us to check the buildroot in distro packages and gnome-continous.
Add an installed test for the destdir-check