Added a SExprPath class for navigating SExp trees.

SExprPath constructs a graph of nodes that leads from
a root SExp to a given SExp. This graph can be then
used to find adjacent SExps.
Checked that solution still builds in MonoDevelop

Also worked on the SExprPath a bit.
Added a test for SExprPath.
Added comments to SExprPath class.

Also changed the type of the root S-expression of the path
to be SExpr instead of NOPList<SExpr>.
Refactored the visual system.

To be able to draw the focused s-expression, visuals need to
store a different appearence settings somewhere. Created
the GraphicsContext class to store the current  graphics
state.
Continued working on focused Sexp support.