Add level option to ve.ui.OutlineItemWidget

Objective:

* Add indentation feature to outline items

Changes:

ve.ui.PagedDialog.js
* Pass level argument through addPage method

ve.ui.Widget.css
* Add styles for non-zero levels of outline items

ve.ui.OutlineItemWidget.js
* Add missing config documentation for icon option
* Add level option, with getLevel/setLevel methods
* Add statically configurable number of levels and level class

Change-Id: Ie24761c64918fdc6cd28070b3cf56743c6288970
Merge "Add level option to ve.ui.OutlineItemWidget"