[greeter] Infographic highlight colors not ideal for different backgrounds

[Michael Hall]

While the selected shades of orange and purple look good on the default wallpaper, and on other purple wallpapers, they don't look good on green, blue, etc wallpapers.

Proposed solution:

* Provide a mapping keyed on different color hues (red, orange, yellow, etc) that contains two sets of "highlight" colors for each hue:
  * 3 shades for the same hue
  * 3 shades of a different hue

* When the wallpaper for the welcome screen is set, find the "average" color like we do on the desktop, and best-match that to the hue keys in the map.
 * Of the 6 defined shades, remove the one that is closest to the wallpaper "average" in hue (this will avoid having a highlight that is too similar to the background)

* Use the remaining 5 hues, alternating, starting with one from the "different hue" set (the set with 3 remaining after dropping one)

Expected result:

On the default wallpaper, which has a purple "average" color, the infographics would have 3 shades of orange and 2 shades of purple.

[Olga Kemmet (olga-kemmet)]

Going tobe part of a bigger redesign.

[Launchpad Janitor (janitor)]

[Expired for unity8 (Ubuntu) because there has been no activity for 60 days.]