OutputPage::getCategories(): Add a possibility to distinguish "normal" and "hidden...

OutputPage::getCategories(): Add a possibility to distinguish "normal" and "hidden" categories

There's currently no way to get, e.g., all categories except the
hidden ones just as text. The OutputPage::getCategories() method
always returns all categories as an array of strings (titles) and
the getCategoryLinks() method returns the result of Linker::link
but with the distinction between "normal" and "hidden" categories.

This change adds a new parameter to OutputPage::getCategories(),
$type, which can be used to define, what categories should be
returned. The default value is "all", which means, that all categories
are returned (the current result of the method). With the value
"normal" and "hidden", the method will return the respective values.

This could be used in I97d7de723fe72da26c7dbde0a559a13704c7099a to
remove the stupid Linker::link() and isset workaround.

Change-Id: Iadda9ae362a21fbee770240234b8f55326219932