Do on-drag special activity via metainfo hints instead of hardcoding

Currently there is a lot of hardcoding related to what is done on component library item drop on navigator. E.g. If a Texture is dropped on Material, a list of possible texture binding options is presented. Similarly, if an image asset is dropped on a node accepting texture drops, it is converted into a texture. There is also special case for drops under Pass, as pass can't accept children, but we want to do the property binding. All this should be possible to handle via item metainfo instead of cpp hardcoding.