RadioButton: change images to match rest of tundra

[bill]

Problem: Button style did not appear cohesive with the rest of the Tundra style.

Current:

Recommendations: Refine button style treatment: (see revised assets)

• a) Flattened the heavy 3D look to a subtle gradient.
• b) Keeping the gradient consistent with other UI elements. (ie. tabs, accordian, etc)
• c) Cleaned up the white artifacts in the gif files so they will display properly on dark backgrounds.
• d) Firefox should be loading PNGs image files not the GIFs.

New:

Taken from IBM Design Leadership recommendations.

[bill]

(In [13566]) Make RadioButton? style appear cohesive with the rest of the Tundra style.

• Flattened the heavy 3D look to a subtle gradient.
• Keeping the gradient consistent with other UI elements. (ie. tabs, accordian, etc)
• Cleaned up the white artifacts in the gif files so they will display properly on dark backgrounds.
• Firefox should be loading PNGs image files not the GIFs.

I'm conflicted about white-artifact cleanup. It makes checkbox look better on a dark background but worse on a white background. But I'm deferring to an expert opinion (see trac ticket).

Fixes #6634.