Add shell_get_file_contents_utf8_sync(), use it instead of gio temporarily

Adding correct annotations to Gio.File.load_contents revealed that gjs
doesn't actually support array+length combinations.  For 3.0 this would
be invasive to fix, so add a method to ShellGlobal which does what
we need.

https://bugzilla.gnome.org/show_bug.cgi?id=646333

src/shell-global.h

@@ -185,6 +185,9 @@ void shell_get_contact_events (TplLogManager *log_manager,
                                guint num_events,
                                GAsyncReadyCallback callback);
 
+char *shell_get_file_contents_utf8_sync (const char *path,
+                                         GError    **error);
+
 G_END_DECLS
 
 #endif /* __SHELL_GLOBAL_H__ */