widgets.text: tweak line limits for better WYSIYWG

We were originally adding two extra characters to allow the line to
actually contains N characters, because Qt wraps at N-1.

We only need one extra character + 1 pixel to get the desired effect.

Closes #242

Reported-by: @wm4 via github.com
Signed-off-by: David Aguilar <davvid@gmail.com>