qdoc: Don't try to add html files for images in .qhp

QDoc adds document nodes as files into the .qhp, including nodes with
image subtype. This will generate a lot of unnecessary warnings for
missing '<image>.png.html' files from qhelpgenerator when running
'make docs'.

This change makes qdoc skip the above step for image nodes to remove
the warnings. The images are still always added to .qch in a separate
step.

Change-Id: Ib07fc8a3ce65f5d4dd700daf00f5f6a36f74a696
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/tools/qdoc/helpprojectwriter.cpp

@@ -375,6 +375,7 @@ bool HelpProjectWriter::generateSection(HelpProject &project,
     case Node::Document: {
         const DocNode *docNode = static_cast<const DocNode*>(node);
         if (docNode->subType() != Node::ExternalPage &&
+                docNode->subType() != Node::Image &&
                 !docNode->fullTitle().isEmpty()) {
 
             if (docNode->subType() != Node::File) {