CorePlugin: Compile with QT_NO_CAST_FROM_ASCII.

- Wrap literals in QLatin1String()/QLatin1Char().
- Extract some string constants avoiding repeated
  QString construction.
- Fix repeated invocation of Container.end() in loops.

Change-Id: If737735507aaf82e53063adda53f54c46418f42f
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>

src/plugins/coreplugin/helpmanager.cpp

@@ -447,7 +447,7 @@ QStringList HelpManagerPrivate::documentationFromInstaller()
             documentationFiles << pathInfo.absoluteFilePath();
         } else if (pathInfo.isDir()) {
             QDir dir(path);
-            foreach (const QFileInfo &fileInfo, dir.entryInfoList(QStringList() << "*.qch",
+            foreach (const QFileInfo &fileInfo, dir.entryInfoList(QStringList(QLatin1String("*.qch")),
                                                               QDir::Files | QDir::Readable)) {
                 documentationFiles << fileInfo.absoluteFilePath();
             }