Implement 'Move "Export Diagram" to File menu'.

The "export all diagrams" menu entry has been refactored to
reuse the diagram filter widget already used by print and print preview.

The diagram filter widget is still named DiagramPrintPage, which is
now misleading and need to be renamed in a further commit.

We use QPointer for m_printSettings to avoid dangled pointers.

CCBUG:109097
FIXED-IN:2.16.80 (KDE Applications 15.07.80)