backends: Disambiguate output mapped to tablet with connector name

In some circumstances, we may end up with outputs with the same vendor/product/serial, in which case we have a hard time finding the right one to map tablets to, since configuration only has these 3 pieces of data.

Add the handling of a 4th argument containing the output name based on the connector (e.g. HDMI-1), so that it can be used to disambiguate the output if necessary.

This only kicks in if there actually are multiple outputs with the same EDID data. A goal of the configuration as it was stored was to remain useful if the user changed how the device is physically connected to the computer, this remains true for the vast majority of users having a single thing of each.