Support addManualDevice

[nallath]

I was reviewing your the plugin in the marketplace, which lead me to also check out the repository. In your readme I found the following:

Do NOT try to add a new "networked printer"! This is only for Ultimaker printers.

This is not the case. A plugin can register itself as being able to detect (and add) printers manually. We try to ensure that we keep as much flexibility as possible within our plugins.

If you check the code that handles the check in the manual IP screen (https://github.com/Ultimaker/Cura/blob/78ecb7ec9a0c1deae6c322a286a13ee70a07e379/cura/Machines/Models/DiscoveredPrintersModel.py#L143-L164) you can see that it asks all the plugins if they can add a manual device given the address. If at that point the plugin responds that it's able, it will be considered in _attemptToManualAddDevice (https://github.com/Ultimaker/Cura/blob/78ecb7ec9a0c1deae6c322a286a13ee70a07e379/cura/Machines/Models/DiscoveredPrintersModel.py#L166-L174)

[Kriechi]

@nallath thanks for reaching out - I was aware that plugins can hook into this mechanism, and the readme statement was supposed to indicate that this plugin does not use this feature... I can see how this might sound confusing in its current "absolute statement" form.

I'm open to revisit the implementation in the future here if this is something we can adopt and possible improve the overall UX for new users.