Don't allow 3D transformations for page turns

They seem to be slightly buggy in WebKit.  Closes #6.
Delay U1db writes

Writing right away was slowing down page back turns and adjustments of
settings.  This delays writes 1 sec, with some amount of intelligence
should mulitple writes come in quickly.  But it isn't a real caching
solution, since there's no guarantee that settings get written, should
the user exit the program.
Improve LineHeight Slider

The thumb becomes grey in the "Default" zone.  Maybe not the best color,
but we can worry about that later.  Also, the "Default" label is
translatable; in English it's now "Auto", which fits better in the
bubble.