Add support for loading decks

[zbanks]

Regression Could be fancier, maybe pre-loaded at the bottom of the UI to select from?

[zbanks]

Added serialize/deserialize to Model. Still needs UI to action load/save.

• serialize should probably take an optional list of nodes to serialize (e.g. a selection)
• Right now VideoNode * -> QString serialization is handled by NodeRegistry which is gross; it should get serialized to a JSON blob and NodeRegistry should delegate to VideoNode to perform both ser/des.
• Need to figure out how/if to handle Output info
• Unclear if Model::serialize should use the unflushed or the "for rendering" state.

[ervanalb]

It would be really great to get partial model load / save implemented soon. Right now I achieve this by cp radiance_state.json cool_deck.json