Handle the block-level submit button as a block option

[nbenn]

Currently a block-level submit button can be enabled by having the block inherit from submit_block. A cleaner solution for this, such as a block attribute that can be set in the base constructor would be nice.

[nbenn]

Functionality-wise one issue that has been raised (in #249) is that for a situation where is_initialized(block), we should auto-submit. This might be the case when default values suffice for initialization or also when restoring the state of a block such that all necessary information is available for eval.