Create MediaWiki specific nodes to contain MW specific rules.

Heading and Preformatted nodes have rules that should only
exist under a document node in MediaWiki.

Two new node types have been created as has a new DropdownTool which
uses these. The MW init options have been changed to use the new
DropdownTool.

Bug: 45295

Change-Id: I3f47e1ae1f5c1415bde58a75385e4bf5f4b8fffc
Merge "Create MediaWiki specific nodes to contain MW specific rules."