Don't reinvent the wheel in SpecialRecentchangeslinked::getExtraOptions

Just call parent::getExtraOptions() first and then simply add the one
additional field. The 'SpecialRecentChangesPanel' hook wasn't called
from SpecialRecentchangeslinked previously, I've wrapped the call in
an if to keep the behavior the same.

Changes this causes:
* If $wgAllowCategorizedRecentChanges is true, Recentchangeslinked
  will include the category filter form as well.
* The target filter on Recentchangeslinked will be displayed at the
  end of the form, below the tag filter.

Change-Id: I4436a63356adb1e0b0daa1aa52c179974a036fa1
Merge "Don't reinvent the wheel in SpecialRecentchangeslinked::getExtraOptions"