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/actionmanager/commandmappings.cpp

@@ -151,7 +151,7 @@ void CommandMappings::finish()
 void CommandMappings::commandChanged(QTreeWidgetItem *current)
 {
     if (!current || !current->data(0, Qt::UserRole).isValid()) {
-        m_page->targetEdit->setText("");
+        m_page->targetEdit->setText(QString());
         m_page->targetEditGroup->setEnabled(false);
         return;
     }