morebits: Add DocumentFragment and CSS class support to QuickForm

Using DocumentFragments means we don't have to remove and replace
container elements all the time, losing their inline CSS styling in the
process.
morebits: Remove padding: 0 from simpleWindow footer buttons

No obvious reason for this, and it creates ugliness in IE10+.
morebits, welcome, shared: Overhaul scrollbox functionality

Previously, scrollboxes didn't resize with their simpleWindow, making them
mostly useless. Now, they do resize with the containing simpleWindow.

The plan is to roll out scrollboxes to more modules, including tag.
tag: Switch to scrollbox; add a link to each template's documentation

Fixes #200.