Fix previews in yellow notes and disabled branches
While it is not necessary to run validate() on insets that do not
produce output (yellow notes and disabled branches), it has to be done
for previewing, since a construct inside the inset may require some
support.
This is done in two steps:
1. in PreviewLoader::dumpPreamble, indicate that a preview is being
prepared. It is not clear why the `for_preview' boolean was not set
before.
2. in Inset(Branch|Note)::validate, always call the parent's validate
method when previewing.
It should have been possible to move the code from 2. to
InsetText::validate, but the weird code in handling
InsetNoteParams::Comment in html makes it difficult.
I took the change and I changed some minor annoyances:
* test comparison with None;
* fixed a region where the indentation was different from all the
others (2 spaces instead of 4);
* replaced xrange with range.
