Fix bug 14267 by adding support for a MediaWiki:Mainpage-nstab.

Fix bug 14267 by adding support for a MediaWiki:Mainpage-nstab.
Additionally, *cough* *cough*:
* Add a Title::isMainPage helper for the fairly common $title->equals( Title::newMainPage() ); test.
* Update wfMessageFallback to also accept an array of message keys instead of requiring them listed as arguments to the function.
* Move the bulk of wfMessageFallback code into Message.php instead of leaving it in GlobalFunctions.php
* Change the wfMessageFallback implementation so that the Message class handles the fallbacks themselves eliminating any side effects caused by the fact that wfEmptyMsg always used usedb=false, language=userlang when one might actually use a different language or usedb setting in the message object that actually returned the text (this may be considered a wfEmptyMsg regression in 1.18).
* Make blank "" message contents fallback like nonexistant messages do.
* Re use the new tabAction array handling used to support mainpage-nstab in the talk and view tabs instead of making wfEmptyMsg calls directly in SkinTemplate.