Speed up Word startup time

Several users reported that Word startup is currently slowed by XWord. This should be optimized.
Here are some improvements that can help on fixing this issue:

• Allow the user to disable auto-connect at startup. Loading the potentially unwanted XWiki for a session is the main speed drawback;
• Load only the spaces and not the pages too. Pages data should be loaded at demand or chached;
• Create a prefetch mechanism;