mediawiki.special.preferences: Support Back/Forward navigation

mediawiki.special.preferences: Support Back/Forward navigation

The current preferences tabbed dialog already uses hashes to identify
different tab sections, but using the browser's Back or Forward
button had no effect.

Added an event listener to the browser 'hashchange' event, so
that when the hash changes (as a result of using Back/Forward), the
display switched to the desired section.

If on the 'hashchange' event the hash is empty, it opens up the first
tab (just like the default when loading plain Special:Preferences
without a hash).

If the hash does not correspond to any section, nothing happens (this
can happen when the user follows an old hash-link to an tab that no
longer exists, in that case we need not do anything as the first tab
is visible by default). Another thing that can cause this is when the
page is already opened and some other component is using the hashes
for something. So we don't want to force a switch to the first tab
either.

Removed duplication of code by introducing function switchPrefTab().

Tested:
- Loading to a hash value
- Switching tabs
- Pressing Back/Forward
in:
* Chrome 19: OK
* Safari 5.1: OK
* Firefox 12: OK
* Opera 11.1: OK
* Opera 11.5: OK
* Opera 11.6: OK

Change-Id: Ic51c3954884680496f76814eed3fe65c9d0daacc