Data-slicer

Changed

- `qtconsole.rich_ipython_widget` -> `qtconsole.rich_jupyter_widget` due to
deprecation

- PIT now uses `utilities.make_slice()` with the `silent=True` option to suppress
a warning.

Changed

- :class:`cmaps.ds_cmap` is now just an empty shell, in order to fix a bug.
In turn, alpha and gamma sliders don't work anymore. The original code is
retained in :class:`cmaps.ds_cmaps_legacy`.

- Created compatibility for PyQt5 by exchanging many instances of `QtGui`
with `QtWidgets`.

Fixed

- ImageItems and GLImageItems not showing up due to a change of how colormaps
work in pyqtgraph.

Added

- changelog.md