Cache completion based on XML Schema/DTD

[angelozerr]

When completion based on XML Schema/DTD is done and XML Schema/DTD comes from the file system:

• the DTD/Schema URI use file system
• the DTD/Schema URI use http but cache is activated

The XMl Schema / DTD is every time parsed and loaded. For a big XML Schema / DTD, there are some trouble:

• completion can be slow.
• if it's too slow, completion returns nothing.

To fix this issue, we should cache the completion (the CMDocument) like we have done for http but we should track all files changed of the XML Schema (which can imported other XML Schema) to clean the cache when the XMl Schema / DTD is modified.