Blocks menu

Another attempt at #8.

Hopefully the blocks should render properly in the drop-down menu now.

> I've written a drop-down menu for inserting scratchblocks, based on suggestions from Lightnin and feedback from the forums.
> 
> It allows the user to select blocks from a list similar to the Scratch 2.0 interface. After selecting a block, it places the cursor inside the first input slot, ready for the user to type into. It also includes a link to the wiki article on the syntax.
> 
> This should help make the scratchblocks feature more discoverable. It still has some minor glitches, but I think these can be fixed later.
> 
> I've also updated the scratchblocks2 plugin to the newest version.