#1705 Allow for nested layouts, with additional rows inside columns.

This adjusts the layout editor to allow for creating nested layouts with rows inside columns.

Drop a block on the left or right of an existing block to split it into a new row. Or if you move the cursor outside the block so it's on the column rather than the block, you can add a column to the existing row rather than creating a new one.

We should probably put this up somewhere for testing to make sure that the layout editor's interpretation of where users want to place blocks is intuitive enough.