[cmv4] Soft-tab deletion should be multiselection aware

[njx]

1. Enter the following text in a file:

foo    = bar;
barg   = baz;
fleeb  = foo;

1. Alt-drag multiple cursors immediately to the left of the three = signs
2. Backspace

Result: the "b" of "fleeb" is deleted, because the soft-tab code is only looking at one of the selections. The heuristic should look at all the lines.

[njx]

Medium priority to me.

[njx]

Fixed, closing.