Define and use macros for Xfconf properties

Define five macros in common/panel-private.h and ensure using them throughout the code:

1. #define PANELS_PROPERTY_PREFIX "/panels"
2. #define PANELS_PROPERTY_BASE PANELS_PROPERTY_PREFIX "/panel-%d"
3. #define PLUGINS_PROPERTY_PREFIX "/plugins"
4. #define PLUGINS_PROPERTY_BASE PLUGINS_PROPERTY_PREFIX "/plugin-%d"
5. #define PLUGIN_IDS_PROPERTY_BASE PANELS_PROPERTY_BASE "/plugin-ids"