CSS: Fix smart tag formatting
CSS: Base64 embed fixes

Fix MIME type detection
Fix Miscellaneous Files ProjectItems
Fix URL decoding
Call GetContainingLine instead of using snapshot

Helps fix ProjectionBuffers
Misc ProjectionBuffer fixes

Properly distinguish between
TextViews and TextBuffers.
Fixes many parts of #131

TODO: Fix CommandTargetBase
CSS: Fix color adornments to work in projections

Enables color previews in CSS embedded
in Markdown or HTML.
CSS: Don't cache CssTrees

Format Document temporarily disposes the
inner TextBuffer, creating a new CssTree.
Therefore, we cannot cache it.
Fixes parts of #131