Replace every use of the foreach macro with a C++11 range-based for loop

This prevents a race condition with Qt5. A foreach loop makes a copy of its
Qt container, increasing the reference count to the container's internal
data. Qt5 often asserts isDetached(), which requires the reference count to
be <= 1. This assertion fails when the foreach loop increases the reference
count at exactly the wrong moment. Using a range-based for loop prevents an
unnecessary copy from being made and ensures this race condition isn't
triggered.

src/core/ComboBoxModel.cpp

@@ -39,7 +39,7 @@ void ComboBoxModel::addItem( const QString& item, PixmapLoader* loader )
 void ComboBoxModel::clear()
 {
 	setRange( 0, 0 );
-	foreach( const Item& i, m_items )
+	for( const Item& i : m_items )
 	{
 		delete i.second;
 	}