Add padded option to layouts

Objectives:

* Add option to layouts to control whether it's padded or not
* Rename "scroll" option to "scrollable" to better fit with it's boolean value

Changes:

ve.ui.Layout.css
* Add styles for scrollable and padded options

ve.ui.StackPanelLayout.js, ve.ui.PagedDialog.js, ve.ui.ReferenceListDialog.js
* Update use of scroll option

ve.ui.PanelLayout.js
* Add padded option
* Rename scroll option to scrollable
* Switch to using a class for scrollable option, rather than hardcoding CSS properties

Change-Id: I50e996b9d2fde361238cde75e09baa5872e12223
Merge "Add padded option to layouts"