Make it possible to apply a certain page template on blank pages

[bertfrees]

"Blank" means: a page without content that is the result of a block with break-before="sheet" or a sequence with break-before="sheet" or break-after="sheet" (see #88). A page that is omitted completely at the end of a sequence due to a break-after="auto" does not fall under this definition. (Note that without this last sentence it would not be possible to differentiate between the two cases, which would make it very hard to support the "old" behavior.)

This can probably be solved most easily by making a new boolean variable available that tells you whether you are on a blank page or not.

Related issue: A new variable that tells you whether you are on a right- or left-hand page: brailleapps/dotify.formatter.impl#34