Clang: Clean up diagnostic widget

* Use a single QLabel - No need for all the QLabels we used. Also, a
  single QLabel enables selecting and copying all the diagnostics, which
  is handy when displayed in the info bar.
* Avoid call to Utils::ToolTip::hideImmediately() if the location is
  clicked from the info bar.
* Simplify code and API

Change-Id: Ib991364e4d6f40ef02dada8ebbb90fe6ff8ae1a1
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangtextmark.cpp

@@ -31,6 +31,7 @@
 #include <utils/icon.h>
 #include <utils/theme/theme.h>
 
+#include <QLayout>
 #include <QString>
 
 namespace ClangCodeModel {
@@ -84,7 +85,11 @@ void ClangTextMark::setIcon(ClangBackEnd::DiagnosticSeverity severity)
 
 bool ClangTextMark::addToolTipContent(QLayout *target)
 {
-    Internal::addToolTipToLayout(m_diagnostic, target, Internal::DisplayHints());
+    using Internal::ClangDiagnosticWidget;
+
+    QWidget *widget = ClangDiagnosticWidget::create({m_diagnostic}, ClangDiagnosticWidget::ToolTip);
+    target->addWidget(widget);
+
     return true;
 }