icontheme: use g_file_load_contents() for symbolic icons

icontheme: use g_file_load_contents() for symbolic icons

When loading a symbolic icon, g_file_get_contents() is currently used
with the icon pathname, to load its SVG data. This won't work when the
icon is not a local file, for instance when a symbolic icon is loaded
from a GFileIcon with a GResource path.
Fortunately GtkIconInfo already holds a GFile, so we can just use
g_file_load_contents() to load the data instead.

https://bugzilla.gnome.org/show_bug.cgi?id=709056