Provide PNG fallbacks for non-SVG web browsers

Uses the exact same fallback mechanism as in
core/skins/common/shared.css.

Aditionally the PNG files are optimized using optipng -o7 -nc (on
purpose since the most basic 32 bit RGBA color type does have the
broadest compatibility compared to, for example, "1 bit palette
with 8 bit alpha channel").

Change-Id: If568552fe47cbc1a150adc3724d5999e9ced644a
Merge "Provide PNG fallbacks for non-SVG web browsers"