Allow override of description

In the latest dev release of FPP, the description in entity_info is hardcoded to be "Create a new custom entity" - see fieldable_panels_pane.inc, line 225. The downstream effect of this in Panopoly is that all of its custom entity types have the same tooltip. In order to allow different tooltips for each custom entity type, it would be nice if entity_get_info checked whether the custom entity type had set this variable, like it does for title, category, icon, etc.