Add the ability to save & load emulator state

[jonathanlloyd]

A common feature in other emulators, this feature allows user to save and load the state of their game session.

Given that the emulator is running
When the user presses "save state"
Then they are given a file to download containing the state of the emulator

Given that the demo is loaded
When the user presses "load state" and selects a valid state file
Then the emulator overwrites its existing state with the data in the file and runs the game from the point at which it was saved