Background on icons, so that they can work in dark color schemes

The icons used in the opencaching.com plugin are dark brown with
transperant background, potentially making them [almost] invisible for
users with dark color schemes. By having a light background, this issue
is prevented. Color is taken from opencaching.com palette.

REVIEW: 107564

M +- -- src/plugins/render/opencachingcom/boat-required.png
M +- -- src/plugins/render/opencachingcom/chirp.png
M +- -- src/plugins/render/opencachingcom/diving.png
M +- -- src/plugins/render/opencachingcom/exclusive.png
M +- -- src/plugins/render/opencachingcom/historic-site.png
M +- -- src/plugins/render/opencachingcom/letterbox.png
M +- -- src/plugins/render/opencachingcom/multi-cache.png
M +- -- src/plugins/render/opencachingcom/night.png
M +- -- src/plugins/render/opencachingcom/puzzle.png
M +- -- src/plugins/render/opencachingcom/traditional.png
M +- -- src/plugins/render/opencachingcom/tree-climbing.png
M +- -- src/plugins/render/opencachingcom/unknown.png
M +- -- src/plugins/render/opencachingcom/uv-light.png
M +- -- src/plugins/render/opencachingcom/verifiable.png
M +- -- src/plugins/render/opencachingcom/virtual.png