QmlDesigner.designModeWidget: fix style of error tooltip

DocumentWarningWidget is now derived from Utils::FakeToolTip and follows
the Qt Creator styling.

src/plugins/qmldesigner/designmodewidget.cpp

@@ -90,13 +90,12 @@ namespace QmlDesigner {
 namespace Internal {
 
 DocumentWarningWidget::DocumentWarningWidget(DesignModeWidget *parent) :
-        QFrame(parent),
+        Utils::FakeToolTip(parent),
         m_errorMessage(new QLabel("Placeholder", this)),
         m_goToError(new QLabel(this)),
         m_designModeWidget(parent)
 {
-    setFrameStyle(QFrame::Panel | QFrame::Raised);
-    setLineWidth(1);
+    setWindowFlags(Qt::Widget); //We only want the visual style from a ToolTip
     setForegroundRole(QPalette::ToolTipText);
     setBackgroundRole(QPalette::ToolTipBase);
     setAutoFillBackground(true);

src/plugins/qmldesigner/designmodewidget.h

@@ -32,6 +32,7 @@
 
 #include <coreplugin/minisplitter.h>
 #include <coreplugin/editormanager/ieditor.h>
+#include <utils/faketooltip.h>
 #include <texteditor/itexteditor.h>
 
 #include <integrationcore.h>
@@ -78,7 +79,7 @@ class DesignMode;
 class DocumentWidget;
 class DesignModeWidget;
 
-class DocumentWarningWidget : public QFrame
+class DocumentWarningWidget : public  Utils::FakeToolTip
 {
     Q_OBJECT
     Q_DISABLE_COPY(DocumentWarningWidget)