Dual Tone Icons for "prev_nav" and "next_nav"

[BeckerWdf]

Fixes: #149
Fixes: #150

[BeckerWdf]

@jasmin261098: Can you pls. review?

[Michael5601]

Thanks for the contribution. The icons look good to me overall except for an admittedly "nitpicky" problem:

The cutout lines are a little bit larger than 2.0px as you can see in the following image where I inserted a black 2.0px line.
As this makes no visible difference in the finished icon I do not insist on changing this but it should be kept in mind for future icons.