Only extend un-serializable fields when updating fixture contents with user input

[ovidiuch]

Only extend un-serializable fields when updating fixture contents with user input (because otherwise you'll get the original fixture if you empty the editor)

How to replicate:

• Use the Flatris playground
• Copy the state of a playing FlatrisGame, after the first tetrimino falls
• Paste it over the fixture of a playing FlatrisGame and the active tetrimino grid from the fixture and the other state will merge