bin: Add method to find by element factory name

A common use case of a dynamically built pipeline is that you want to (conditionally) find a certain element, e.g. the rtpbins in a uridecodebin. If that element has a fixed name inside its parent bin (and only has a single instance) this can be easily done by gst_bin_get_by_name().

If there are multiple instances of the element however, you can only use gst_bin_iterate_all_by_interface(), but this doesn't work if you don't have the specific GType (which is often the case, due to plugins being dynamically loaded). As such, a last fallback is to use the well-known name of the element factory (in case of our example, "rtpbin").