Improve selection display

[RodrigoSantiago]

When there is a selection with empty lines, the result may confuse the user :

Some text editors use half-characters (github web editor) :

And others fill the entire line (intellj) :

[JordanMartinez]

I agree. I've actually been annoyed by that myself but never thought too much about it.

Off the top of my head, I think the half-character approach would be easiest.

[JordanMartinez]

After having a look, this would require more-than-minor refactoring

[JordanMartinez]

Personally, I like the full-line approach best.