UI emprovements to new connection's XMLRPC URL endpoint.

Currently, when creating a new connection, the user has to enter the XMLRPC endpoint of the XWiki instance he wants to connect to.

For new users, this is generally hard to understand, since it's not that well documented and most of them don't have a basic understanding of XMLRPC.

For this reason, we should have some UI emprovements that make this process simpler, either by extracting and computing the endpoint automatically from the URL of the wiki that the user normally uses on his web browser, or trough a process that computes the endpoint based on segments provided by the user.

Any chosen approach should still offer the possibility to manually enter or modify the endpoint for cases where it needs some tweaking to make it work.