Removed custom code inside KJotsWidget::print, which did the pagination and painting:
QTextDocument::print() is quite capable of it on its own.
Make Qt also responsible for margins, document placement, etc.

Make QPrintPreviewDialog actually work: it requires to connect to paintRequested signal.
For this purpose, made KJotsWidget::print a slot.

Unify QPrinter creation code, add a setupPrinter method for this purpose.
Note: HighResolution property was removed, as it results in weird-looking bold fonts.

1. Print Preview works again
2. Try to select some text and press Print Preview -> it previews selection only
3. Print also works (tried only virtual PDF printer though)