Fix the slider to request a non-zero length, and make its width a

Fix the slider to request a non-zero length, and make its width a
little larger.

The text widget can now be meaningfully associated with a property; in
PUI, it's "value" isn't the same thing as its label, but we can hack
things to treat them symmetrically.

Commit an experimental "live" property that can be set on widgets to
cause them to update their values every frame.  This works great for
text widgets, as above.  Note that this synchronization is input-only:
no support is provided (or needed -- the GUI only changes when the
user does something) for writing those properties out every frame.