- Remove IHaskell.Display.Widgets.Common
- Partially refactor Button implementation
- Stuck with ToJSON instance for Widget

Dropdown and Output Widgets

- Add explicit export list
- Rename to DropdownWidget
- Add selection handling

- More features may be added

- Add dropdown widget and some functions to manipulate it
- Requires the ability to update the internal stored widget without
  sending a message to the frontend.

- Add support for `clear_output`
- Rename `setOutput` to `appendOutput`

- Add Output Widget to ihaskell-widgets
- Add support for sending `display_data` messages as custom messages.

Backbone widgets

- Move update and modify to Common
- Move str :: String -> String to Common
- Rename ImageInt to PosInt
- Move PosInt to Common
- Correct some comments erroneously referring to Buttons

Add ImageWidget

Use readFile from ByteString to read binary files

- Send [method .= custom] messages via widgetSendCustom
- Send arbitrary JSON messages via widgetSendValue
- Refactor IHaskell.Eval.Widgets, tidy up handleMessage

- Reformat according to hindent
- Remove String.hs

Reading a file that might contain PNG data using `System.IO.readFile`
causes `hGetContents: invalid argument (invalid byte sequence)` due to
file contents not being valid text under the current system locale.
This fixes loading diagrams into Jupyter notebooks.

- All four string widgets complete.
- Switch to using 'Text' everywhere. How'd String crop up?

- All four widgets work. TextWidget still missing `on_submit`.
- Still a lot of errors in the webconsole. Don't cause trouble though.

- HTMLWidget and LatexWidget work fine.
- TextWidget still doesn't work.
- TextareaWidget not implemented yet.

* All widgets raise a lot of errors in the webconsole.

It is not required to re-update the widget inside the kernel state every
now and then, as it contains IORefs, which stay up-to-date.

- Make hlint happy.
- Reformat using hindent.
- Add explanatory comments in some places.
- Use Control.Monad.foldM for IHaskell.Eval.Widgets.widgetHandler

- Return of the newlines: Add all newlines once again
- Provide better comments
- Remove `sender`
- Consistently use set and get prefixed functions

Catch SomeException in IHaskell.Eval.Evaluate.capturedIO
Click handlers can now use error and undefined.
All exceptions are caught, which might also cause problems.

* Add explicit export list
* Provide direct access to click handlers
* Rename getButtonText to getButtonLabel

It just works.

Separate out publish to IHaskell.Publish
Replace publish with publishResult, which also requires more arguments.

Previously, the kernel would die when opening a notebook with widgets
as there is no widget stored in the kernel when it starts.

It wasn't even working. The displays were being generated due to the
sendWidgetView call from display for Button.

All functions now accept a Button as their first argument.

Evaluate accumulated widget messages in the Ghc Monad, after a comm_msg
recieved from the frontend is handled.

- Move dupHeader to IHaskell.Types
- Move handleMessage and widgetHandler to IHaskell.Eval.Widgets
- Make handleMessage and widgetHandler more flexible

Buttons were seen in the notebook. It seems that an empty label was the
culprit, even though IPython somehow manages to accept empty strings.

Still no visible outputs.