Adds a version of template_ID that can be used for non-ID properties. The property to search for and the collection to search in has to be passed to it. Like template_ID it also takes arguments to define a 'new' and 'unlink' operator. They will be displayed as icon-only buttons then. Also added a version that can display preview thumbnails. Had to do some additional changes to make text-buttons support displaying/modifying empty RNA properties. This will be needed for workspaces, see D2451. Reviewed By: campbellbarton Differential Revision: https://developer.blender.org/D2666