ENH: Add toggle visible button

The selected categories under the visibility tab can now be toggled
using the "Toggle Visible" button.
TST: Add tests to support toggle visibility button
ENH: Add callback to toggle category visibility

This callback will hide the categories that are visible and show the
categories that are hidden. With this change we needed a way to refer to
the table with the visibility widgets, hence a new identifier has been
added to it.

Fixes #145.
MAINT: Add note to ChangeLog.md re toggle visible
ENH: Add horizontal line to divide the widgets

An horizontal line now divides the elements under the Visibility and
Scaling tabs, this line separates the "per category" controllers from
the "global controllers".

The test suite was updated accordingly.
ENH: Change button name to Invert Selected

Thanks for the suggestion @teravest
Merge pull request #253 from ElDeveloper/issue-145

Add toggle visibility button