in_bounds? for TextBlock

TextBlock's claim to be clickable, but weren't doing correct bounds
checking.

This commit implements the bounds checking in the backend class and
delegates to it from the DSL.
Safer in_bounds? check

Original version of the bounds checking was, for whatever strange reason
made sense to me at the time, modifying the bounds we got back to do a
contains? check. This is odd and potentially hazardous (if layouts
didn't return a clean copy of bounds).

Additionally in reviewing it, I realized the logic fit much more cleanly
within the FittedTextLayout class itself, so I moved it along with
removing the strangeness.
Merge pull request #662 from shoes/para_clicking

in_bounds? for TextBlock