gs-application: add a local file to define personal customizations.
CSS files of GNOME Software are built-in, so there is no way to put your
own customizations to create an specific App Store. This patch adds a
local css file at
/usr/share/gnome-software/ to enable non-default CSS
customizations. Important: This method is sensible by theme changes.
Signed-off-by: Julio Faracco firstname.lastname@example.org