Initial structure view (no icons or pretty names)
Denote paths in AST and Psi

Add a node for "path" elements (complex IDs like "foo.bar") since they
should be treated as a unit in an number of contexts (our immediate
need is extracting the text of the whole path for the structure
view)

A side effect of this change is that it actually fixes an issue with
auto-insert of close tags (previously, {{#foo.bar}} would incorrectly
get a close tag of {{/foo}} inserted, rather than the desired
{{/foo.bar}})
Implement structure view names and icons

Associates a logical name with each node in the structure view tree,
and adds an initial set of icons (these still need a bit of work).

Also builds out the Psi tree a bit to support this, cleaning up some of
the existing classes and adding elements for Paths ("foo/bar/baz") and
Data ("@data").
Update structure view icons

Includes dark theme and retina icons
"Autoscroll from source" in structure view