Use the system GTK icon set for Pinta icons

[isf63]

Description Some versions ago, Pinta used a colorful custom iconset for its icons. This can be seen in the screenshot here: https://en.wikipedia.org/wiki/Pinta_(software) Since, the developers have switched to system-supplied icons, but only in B&W.

I personally find the B&W/symbolic icons hard to search through. It would be great if Pinta used the icon theme set in the GTK configuration.