Clarify the purpose of the bin history menu

[faucomte97]

Task Description

Rapid Router has a "bin" feature, allowing the player to drag and drop blocks to the bin to discard them.
After that, the player may click the bin to reveal the "bin history" i.e. what's been discarded since the start of the level, allowing them to retrieve past blocks. This shows up in a grey side bar on the right hand side of blocks space.

We feel the "bin history" might not be immediately obvious so we need to make it clearer.

Acceptance Criteria

• Add a piece of text inside the grey bar to label it.
• "Bin history" is an option - confirm this with @lauracumming.
• The placement of the text should probably be at the top of the grey bar - confirm this with @lauracumming.
• BONUS: The "bin history" always seems to contain a "Start" block. This is an issue because it allows players to drag a new Start block to the game which breaks it. Bonus points if you can remove that Start block from the bin history.