New overlay method for the KMail and Akregator dock icons.
Works quite similar to the one in the battery applet, but because we're not Plasma, it's still blue-on-white with most color schemes.

Tested on all shipped color schemes and three different icons.

Works fine everywhere except for Obsidian Coast and Honeycomb - those lack contrast between link background and link foreground, little that we can do here except switching to KColorScheme's "Normal" colors (but that doesn't look as nice, so let's rather have the tradeoff).

Hope you like it.