Add contrast and brightness controls to Preferences

[ejeschke]

This adds contrast and brightness (bias) sliders to the channel preferences, offering another way to set these values that may be more intuitive for some users.

This also changes the ginga.RGBMap object to remove the shift_array and color_array preferences. These are now replaced by color_map_invert, color_map_rot_pct, contrast and brightness. The former were array values and the latter are scalars; this makes the channel settings file a little easier to read and parse. This is backward incompatible from an API standpoint to the RGBMap object with regard to these arrays. However, it removes some problematic issues with managing color preferences.

For end users, the effect is simply that an old channel settings file will not load any saved contrast, inverted or rotated color settings. Once they save any channel settings again the new keywords will be saved and take effect on the next restart.

[ejeschke]

@pllim, when you get a chance, try out this PR and checkout the differences in the channel settings (Preferences plugin). Let me know what you think.

[ejeschke]

@pllim, would you have a chance to check this before SciPy? I'd like to include this merge this into dev for a v5.0.0 release this fall. Perhaps we can discuss that release at the conference.

[pllim]

Sorry, it's on my queue but things keep popping up. I'll bump this up for next week.

[ejeschke]

Rebased on top of latest release

[ejeschke]

Thank you, @pllim!