[2SP] Remove custom BlockRegistry from opendigitizer , use standard one from gr4

[drslebedev]

opendigitizer includes a custom implementation of the block registry that replicates the functionality of BlockRegistry from gr4.

This custom implementation should be removed, and the gr4 implementation should be used exclusively.

[frankosterfeld]

I think especially for handling of remote flowgraphs it would be useful to have a block type "meta" description (describing available block types, their properties, ports, etc.) that's not tied to locally available GR4 blocks. For that I'd find it desirable to have a separate data structure that doesn't require that the described block can be instantiated locally.
For remote graphs, the worker would send these descriptions to the UI, which the UI would then use in the remote graph editor.

One could consider to move the "meta type" structure up to GR4, and extend GR4's BlockRegistry provide these meta types. Right now it's focused around creating instances, instead of describing their properties without instantiating them.