Refactor block selection

Block selection was "broken" when using tabs, or rather
incomplete: It treated tabs as normal characters, which
has shown to be unexpected by people using tabs in code.

The new implementation has a vastly improved find scope
as well. In addition, creating a blog selection with
mouse or keyboard feels a lot more solid now, as the
actual selection is detached from possible valid cursor
positions.

Task-number: QTCREATORBUG-1541