Adds a dialog for adding and moving blocks, activated when the user pressed Space/Enter
with a blocktype focused, or activates the new "Move block" button. The user can select
which cell (row/column) to put the block in and where in the cell to place the new
block.
The "Move block" button is hidden unless it gains keyboard focus meaning the interface isn't
any more cluttered for mouse users.

Change-Id: I9a3ced66c0947756f1f80b0dacff82de201c2142
Signed-off-by: Jono Mingard <jonom@catalyst.net.nz>