Support add + remove blocks to a stack

[DivadNojnarg]

Will fix #15

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[DivadNojnarg]

Still todo:

• Add confirm button to add block.
• Fix missing data rendering for added block.
• Be able to remove a block.

[DivadNojnarg]

@nbenn : I noticed that when I remove a block from a stack with only 1 block, I do get:

list()
attr(,"name")
[1] "pzbsytnpwvtgitb"
attr(,"class")
[1] "stack"

which seems consistent. Then for some reasons, the stack seems to automatically add a new block to itself.

[DivadNojnarg]

So far: we can add a new block to a stack with a data_block. We can remove that new block and repeat again. What does not work: empty the stack and add a new block: for some reasons the stack refills and this mess up the process. I also don't check when we remove a data block and there are other blocks downstream (this should raise an error in the UI saying that the data block is mandatory)...

[DivadNojnarg]

Progress:

• Can add block to empty stack.
• Can add block to any existing stack.
• Can remove any block, except a data block when there are downstreams block.
• Can remove data block when it is the only block remaining.

@nbenn TO DO: add server testing (testServer) to improve coverage.

[DivadNojnarg]

I am done with this one @nbenn and @christophsax. Feel free to check. I'll improve the look and feel with the second ongoing pull request.