Add option for custom Tree Supports

[CodingDutchman]

Thank you so much for the Custom Supports plugin. It can really be a life saver. However, sometimes I need a support in a spot that is not over the build plate and I don't want to rotate the model. It would be amazing if placing of a custom support block at a spot would make Cura work as hard as it can to support that block with whatever the current support settings are (be they regular or tree supports, "touching build plate" or "everywhere").