Add a "Export to PDF" toolbar button to the QPrintPreviewDialog

QTBUG-65554 inspired the addition to the QPrintPreviewDialog toolbar of a "Export to PDF" action. In applications that provide a "print preview" menu action this will allow users to generate PDF documents that should be as identical across platforms as Qt's internal PDF producer allows. This producer is not currently accessible via a standard UI on Mac (nor MS Windows?) and has the additional advantage that it is capable to embed fonts even if the Freetype fontengine is used (which leads to PDF files in which text can be selected and which can be much smaller).

The change itself is trivial: the logic for exporting to PDF is already in the `_q_print()` slot (can be moved to a new slot and called from `_q_print()`). The most difficult aspect was finding an appropriate icon

I can put up my patch for review if you think the feature is useful (and the change small enough not to have to ponder it for weeks ^^)