Handle OS X dark mode

[brettbode]

When using dark mode on recent OS X versions the button icons on the main window look very poor. Look into handling dark mode better, possibly with alternate icons.

[brettbode]

The lists in the orbital surface window are very difficult to read in dark mode.

[brettbode]

Code checked in to handle the AO and MO lists in the surface window.
Need to work on the axis labels for the energy and frequency plots.

[brettbode]

Checked in fixes for the frequency and energy plot windows. They both will change with dark mode correctly. Copy and exporting graphs results in the normal light mode version regardless of display mode.

This addresses the issues I know about where dark mode display is "broken". Still to consider are modifications to the toolbar images (particularly the View image does not look very good in dark mode) and inverting the background color in the main display with dark mode (if it is the default). A preference to disable the behavior should also be added.

[brettbode]

Added support to the molecule display window and tweaked the view image to look better in dark mode.