Add ihaskell-graphviz to display graphs.

Requires 'Graphviz' installed.

Update .travis.yml

ihaskell-widgets: Fix more warnings

This file was giving a strange error/warning that had to be disabled.

After investigating this issue (https://ghc.haskell.org/trac/ghc/ticket/15601)
it turns out this was due to an wart in the way type level lists are handled
in GHC. Fortunately a workaround is available allowing the warning to be
re-enabled.

Fix more warnings!

GHC 8.4 and later have the `-Wpartial-fields` warning flag, so add it and
fix the warnings.

Turn on and fix more warnings

ihaskell-widgets: Make it compile with ghc 8.4

Fixing these warnings required changes to the file names of the `MessageHeader`
struct which makes this commit an API change. The version in the cabal file
has been bumped accordingly.

-Wall the things

Add `-Wunused-do-bind -Wunused-local-binds -Wunused-top-binds -Wunused-matches`
and fix all the things.

Add `-Wmissing-signatures` to `ghc-options`.

Add `-Wincomplete-patterns` to the `ghc-options` field of the cabal file
and fix all warnings.

GHC 8.4 allows value level operators (in this case list append, '++') to
be used at the type level, so that the singletons package for GHC 8.4
uses this operator whereas previous versions of singletons defined a ':++'
operator for type level append.

Drop support for GHC < 8.0

Support GHC 8.6