Add XmlSerializer cache to fix memory leaks.

[pkindruk]

Implemented 2nd option I suggested in #356

[eriove]

Not a maintainer, just an user. Would it be safer to use a ConcurrentDictionary? Before this change the write-operation was thread safe. Even though I can not really thing of a reason to save it multiple times it is an easy change to make to avoid running in to tricky bugs.

[pkindruk]

@eriove good point on thread safety, thanks.

Normally you do serialize/deserialize on UI thread since XmlLayoutSerializer uses DockingManager.Layout dependency property. However all those implementation of IXmlSerializable are public and can be invoked in custom ways, so thread safety might be needed.