[Diagram Editor] Allow user to set display text for buffers

[mxgrey]

Currently buffer elements are always just labeled with the text "Buffer", making it unclear what the purpose of the buffer is.

Users should be able to set the display text of buffer elements. The display text can then be used like a variable name for the buffer, which would make the purpose of the buffer more clear.

[mxgrey]

Along with this, it would be good if we can introduce a new visual shape for buffers, similar to what #219 has done for the different forking operations.