Add GUI tests for Backspace key in MSA

The following scenarios should be tested:

1. User selects a single base in an alignment.
User presses "Space" a few times.
User presses "Backspace" as many times as "Space".
Expected state: after each pressing "Backspace" the selection moves to the right by one base. Eventually the alignment returns to its initial state. Selection always keeps its size.

2. The same as previous, but initial selection is a rectangle with size more than 1x1 and the rectangle's right side is the alignment's end.

3. User opens alignment with a few successive empty columns.
User selects the first non-empty column after the empty region.
User presses "Backspace" a few times.
Expected state: empty columns preceding the region have been removed

4. User opens an alignment having the area of trailing gaps.
User selects some sub-area in trailing gaps.
User presses "Backspace".
Expected state: alignment has not changed, "undo" and "redo" buttons are disabled.

5. Users selects a region that is wider than a preceding gaps region.
User presses "Backspace".
Expected state: All the preceding gaps has been removed, selection has moved to the same distance as removed region width.