Add PDF page import to the scripting API

qet.addPdfPage() renders one page of a PDF file to an image and
places it, through the same QPdfDocument::render() call, white-
background compositing (a transparent page would otherwise show
whatever is under it, unlike every other placed image) and
DiagramImageItem/AddGraphicsObjectCommand underneath as the "add PDF"
toolbar action's own file/page-selection dialogs.

Only reachable in a build with the QtPdf module (Qt >= 6.4) -- some
Qt6 distributions omit it entirely (see diagrameventaddpdf.h). The
method is still always declared and compiled, guarded internally
instead of with the class itself: a script asking whether qet.addPdfPage
exists must never get "not a function" for a reason it has no way to
discover. Refuses with a clear reason when the module is missing, the
page number is out of range, the file cannot be loaded, or the
resulting render is degenerate.

Verified against a real 2-page PDF (this session installed qt6-pdf-dev,
which was missing here) that the two rendered pages differ in content
and that dpi scales the rendered pixel size linearly, not just that the
call returns something.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sources/scripting/qetscriptapi.h

@@ -499,6 +499,8 @@ class QetScriptApi : public QObject
 		Q_INVOKABLE bool setImageScale(int folioIndex, int imageIndex, double factor);
 		Q_INVOKABLE bool setImageRotation(int folioIndex, int imageIndex, double angle);
 		Q_INVOKABLE bool deleteImage(int folioIndex, int imageIndex);
+		Q_INVOKABLE int addPdfPage(int folioIndex, const QString &pdfPath, int pageNumber,
+								   int dpi, double x, double y);
 
 		// -- the text fields shown on a symbol (label, terminal names, ...) --
 		Q_INVOKABLE QStringList elementTexts(int folioIndex, const QString &elementUuid) const;