QmakeProjectManager: Do not keep an IDocument in QmakePriFile

Instead, we keep the IDocuments as "extra project files" in the Project
class, like the other project managers do it.
This has two advantages:
    - The document is no longer created in a parser thread
      callback, improving Qt Creator responsiveness while
      loading a project.
    - The IDocuments no longer get needlessly destroyed
      and re-created on a re-parse.
This is relevant because adding these objects to the DocumentManager
results in the creation of file watchers, which is expensive.

Task-number: QTCREATORBUG-18533
Change-Id: I49c03377974e6b33340234dbabbbd82b8d0c827c
Reviewed-by: hjk <hjk@qt.io>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -259,6 +259,15 @@ void QmakeBuildSystem::updateCodeModels()
     updateQmlJSCodeModel();
 }
 
+void QmakeBuildSystem::updateDocuments()
+{
+    QVector<FilePath> projectDocuments;
+    project()->rootProjectNode()->forEachProjectNode([&projectDocuments](const ProjectNode *n) {
+        projectDocuments << n->filePath();
+    });
+    project()->setExtraProjectFiles(projectDocuments);
+}
+
 void QmakeBuildSystem::updateCppCodeModel()
 {
     m_toolChainWarnings.clear();
@@ -528,6 +537,7 @@ void QmakeBuildSystem::decrementPendingEvaluateFutures()
             m_asyncUpdateState = Base;
             updateBuildSystemData();
             updateCodeModels();
+            updateDocuments();
             target()->updateDefaultDeployConfigurations();
             m_guard.markAsSuccess(); // Qmake always returns (some) data, even when it failed:-)
             m_guard = {}; // This triggers emitParsingFinished by destroying the previous guard.