add land, ocean and coast color to lsm

This PR changes the value of the lsm formatoption to a dictionary. Old values are, however, still supported.

In this PR, we additionally add widgets for the psyplot-GUI for the following formatoptions, lsm, xgrid and ygrid (see #1).

linked to https://github.com/psyplot/psy-view/issues/26

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes