Parse json shelf descritions, build and update the RenderMan shelf.
All shelf descriptions are contained in one or more shelf.json file in the usual
locations:
. ~/rfm/config/shelf.json
. $RFM_SHOT_PATH/config/shelf.json
. $RFM_SHOW_PATH/config/shelf.json
. $RFM_SITE_PATH/config/shelf.json
Each file will be considered to add items to the shelf.
When the shelf is rebuilt (because the plugin being loaded is newer than the
shelf version), all buttons that were manually added to the RenderMan shelf
will be removed and only those described through one of the json files will be
restored.