Projects mode: Disable "Import" button if not supported by project

Task-number: QTCREATORBUG-17399
Change-Id: I79b64298c902776282d54d70efdc0eef03415a51
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/projectwindow.cpp

@@ -391,6 +391,9 @@ public:
         m_importBuild = new QPushButton(ProjectWindow::tr("Import Existing Build..."));
         connect(m_importBuild, &QPushButton::clicked,
                 this, &SelectorModel::handleImportBuild);
+        connect(sessionManager, &SessionManager::startupProjectChanged, this, [this](Project *project) {
+            m_importBuild->setEnabled(project && project->projectImporter());
+        });
 
         m_manageKits = new QPushButton(ProjectWindow::tr("Manage Kits..."));
         connect(m_manageKits, &QPushButton::clicked,