Use ‘refuses first responder’ property

We still need the subclass for rendering the control as active, but the less we override the better.
ARC: Update OakTextView framework
Gutter height now accounts for horizontal scrollbar

Previously the gutter and the text view were set to have the same height (via constraints). Though when the horizontal scrollbar is visible then the gutter should technically be made a little taller to account for the extra bottom margin. Not adding this space was causing problems.

Fixes #773.