Allow blocks to be moved past the beginning and end of the grid

[rasmus-storjohann]

Sometimes I have alignments that happen to sit right at the very left side of the grid. If I want to move one of the sequences to the left, I first have to select all the sequences, move them to the right to make room, then move the relevant sequence to the left.

Solution: If a sequence is sitting to the far left of the grid and I pull it to the left, then all the other sequences should be shifted to the right, which has the same effect. The symmetric case on the right of the grid too, of course.