Allow template provider to configure the action to execute upon document creation

See proposal and vote in this thread:
http://xwiki.markmail.org/thread/spbeqweqdrkuthbj .

For the moment, we'd implement the "edit" action (which will be the default and preserve the current behaviour) and the "save & edit" action, which will actually save the document before displaying the edit form for it.

In the future, the mechanism can be extended with a "custom" option, where the user can fill in his own velocity script, as well as with a "save & view" action, if we find it necessary.