Change the default link label format to show titles instead of page names

With the introduction of Nested Documents, we will be having more and more links to WebHome documents.

Currently, XWiki`s rendering uses page names by default when rendering the link labels in the rendered HTML content. This results in a big usability problem since most links will have the useless label WebHome which does not differentiate between links and confuses users.

We`ve had this problem in the past as well, but now it`s growing to the point where we can`t ignore it.

The major blocker for changing the default, ATM, is the performance hit that would be received from rendering document titles on each document display. We are already suffering from this since we`re using document titles explicitly in many places in the UI and applications, but if we do it in the content as well, the impact will grow.

One solution would be to implement a document titles cache, as XWIKI-1253 proposes. Doing so would allow us to switch to titles by default.