VIM-632 Restored visual block mode that was broken due to multiple carets support

Vim actions with multiple carets in modes other than visual block are
not supported yet. It will be a separate feature. The idea behind this
commit is to fix the regression in visual block mode.

Bounds of visual selection are no longer reversed if they are
overlapped, this fixes expanding the block selection in all directions.

IdeaVim no longer uses SelectionModel.hasBlockSelection() since it
always returns 'true' now.

There are a couple of places where we remove secondary carets when
moving the primary caret or re-setting visual mode. It may be
incompatible with the forthcoming support for multi-caret Vim actions.