Core/ProjectExplorer: Deprecate QDom in favour of QXmlStream-classes.

Rewrite PersistentSettingsReader/Write and CommandsFile to
use QXmlStreamReader/Writer.

Files will now be written out using native line endings and
contain proper XML version information.
Keyboard schemes will contain empty shortcut elements for empty
elements.

Rubber-stamped-by: con

src/plugins/coreplugin/uniqueidmanager.cpp

@@ -63,7 +63,7 @@ int UniqueIDManager::uniqueIdentifier(const Id &id)
     return uid;
 }
 
-QString UniqueIDManager::stringForUniqueIdentifier(int uid)
+QString UniqueIDManager::stringForUniqueIdentifier(int uid) const
 {
     return m_uniqueIdentifiers.key(uid);
 }