\SMW\HooksLoader and \SMW\MediaWikiHook

Use a HooksLoader to register hooks and enable to test implementations
under "normal" unit test conditions.

This can be move into something like [1] at a later point.

[1] $this->globals['wgHooks']['BeforePageDisplay'][] = function ( OutputPage &$outputPage, Skin &$skin ) {
 return isset( $this ) ? $this->hookLoader->register( new BeforePageDisplay( $outputPage, $skin ) )->process() : true;
}

Change-Id: Id76fe06fe5e774a17ad0f7614b7027752a3770c3
Merge "\SMW\HooksLoader and \SMW\MediaWikiHook"