Tightening TextBlock before reconciling tests

The new TextBlockFitter radically changes how the TextBlock behaves, so the
tests are going to get a serious revamp. This commit removes some unneeded bits
before doing that work of reconciling things to the new state.
Rebuild TextBlock tests

Majority of the spec was really testing the painter anyway, so:

* Moved painting tests over to spec for TextBlockPainter, and will fix later
* Rewrote tests on TextBlock from top to bottom
Naive restoration of cursor in text blocks

Sets the cursor and manages it fine, except it doesn't understand about the
multiple text layouts. We need to determine which layout the cursor should show
up in, and properly place it based on that, rather than letting the last layout
get the cursor set.
Naive re-enabling of links

Only works with first pass on rendering a text block.
Also doesn't span multiple text layouts.