extensions-tool: Split out get_extension_property() helper

For the prefs command, we first fetch the extension info to check
whether the extension exists and actually has preferences. This
pattern can be useful for other commands and properties, so split
out a generic helper function.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2391

subprojects/extensions-tool/src/common.h

@@ -54,6 +54,10 @@ void print_extension_info (GVariantDict  *info,
                            DisplayFormat  format);
 
 GDBusProxy *get_shell_proxy (GError **error);
+GVariant   *get_extension_property (GDBusProxy *proxy,
+                                    const char *uuid,
+                                    const char *property);
+
 GSettings  *get_shell_settings (void);
 
 gboolean settings_list_add (GSettings  *settings,