Refactor rendertree to simplify context handling

Get rid of the separate renderContext stack and instead have a parent
pointer on renderer nodes. This lets us walk back up the render tree to
resolve context references
CodeMirror updates
Allow a default value to be specified for WikiRenderTree.getContextVariable
Add namespace support to the element renderer

This allows us to create SVG elements with the correct namespace, which
allows us to have inline SVG images (coming up next)
Refactor the core icons to be inline SVG fragments

This allows us to style the images with CSS, which means we finally get
hover effects on our toolbar buttons