Fieldsets and default sort keys

Objective:
* Add default sort key field to meta dialog
* Replace PagePanelLayout with a generic panel containing one or more FieldsetLayout elements

Changes:

*.php
* Added/removed file links

ve.dm.MWDefaultSortMetaItem.js
* Added getContent method

ve.dm.MetaItem.js
* Added replaceWith method

ve.dm.MetaList.js
* Allow insertion at the end by omitting offset and index

ve.dm.MWMetaDialog.js
* Added default sort key field
* Put category and default sort fields inside fieldsets
* Added loading/saving default sort key

ve.ui.PagedLayout.js
* Changed class used for pages to generic panel layout

ve.ui.PagePanelLayout
* Moved title/icon stuff to field set

ve.ui.FieldsetLayout.js
* New class, adds fieldset with legend

ve.ui.StackPanelLayout.js
* Moved up to the layouts directory

ve.ui.Dialog.css
* Moved style for paged panel from layout stylesheet

ve.ui.Layout.css
* Added styles for fieldsets

ve.ui.Widget.css
* Adjusted margins of input label widgets

ve.ui.MWCategoryWidget.js, ve.ui.MWCategoryPopupWidget.js
* Added setDefaultSortKey method

Change-Id: I979f5e3f08a688790c9f54086206ed1999af13ea
Merge "Fieldsets and default sort keys"