Help: Fix compile for Qt5.5

Change-Id: I8b8063043d5a6be2dc8f9ce1e2e9e36646959050
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Robert Loehning <robert.loehning@qt.io>

src/plugins/help/helpviewer.cpp

@@ -109,7 +109,7 @@ QString HelpViewer::mimeFromUrl(const QUrl &url)
             return QLatin1String(e->mimeType);
         ++e;
     }
-    return "";
+    return QString();
 }
 
 bool HelpViewer::launchWithExternalApp(const QUrl &url)

src/plugins/help/helpwidget.cpp

@@ -396,9 +396,9 @@ QString HelpWidget::sideBarSettingsKey() const
 {
     switch (m_style) {
     case ModeWidget:
-        return kModeSideBarSettingsKey;
+        return QString(kModeSideBarSettingsKey);
     case ExternalWindow:
-        return kWindowSideBarSettingsKey;
+        return QString(kWindowSideBarSettingsKey);
     case SideBarWidget:
         QTC_CHECK(false);
         break;

src/plugins/help/localhelpmanager.cpp

@@ -79,16 +79,16 @@ static const int kDefaultFallbackFontSize = 14;
 static QString defaultFallbackFontFamily()
 {
     if (Utils::HostOsInfo::isMacHost())
-        return "Helvetica";
+        return QString("Helvetica");
     if (Utils::HostOsInfo::isAnyUnixHost())
-        return "sans-serif";
-    return "Arial";
+        return QString("sans-serif");
+    return QString("Arial");
 }
 
 static QString defaultFallbackFontStyleName(const QString &fontFamily)
 {
     const QStringList styles = QFontDatabase().styles(fontFamily);
-    QTC_ASSERT(!styles.isEmpty(), return "Regular");
+    QTC_ASSERT(!styles.isEmpty(), return QString("Regular"));
     return styles.first();
 }