Make fixed icons as png instead of svg

[m4u9]

It would simplify the maintenance by a lot to have fixed icons such as 16, 22, 24, 48 etc use png directly in the source and have a scalable folder with svg to use as template.
With the current structure we need to touch every single svg for each size and it's a lot of work.

I've done a few tests with the monochrome panel icons and it would work