Determine required blocks for created levels

[evemartin]

At the moment, it is possible to save a custom created level without the necessary blocks to solve it, making the saved level impossible to solve. For example:

• you can save a level with multiple houses without the "deliver" block
• you can save a level with a turn in the road without the "turn left" or "turn right" blocks

We need to determine when/with what maps the level creator should be prompted to add more blocks and what blocks they should be prompted to add.