Add a new and more expressive XWiki.copyWiki method that allows overriding existing pages

The current XWiki.copyWiki method does not override by default existing pages in the destination and neither does it have any "override" parameter.

The same can be applied to the XWiki.copySpaceBetweenWikis which currently has the same (do not override) behavior.

This is an issue when certain clients (like XEM) assume that override is enabled by default and the copy operation can be incomplete.