This patch adds edit-opacity icons requested by Okular for D15580.
I'm not quite sure about the naming, and even if the Qt SVG renderer can handle this transparency. I also considered a style with a checkerboard pattern, which is the symbol for transparency e.g. in GIMP, but I felt this is not recognizable enough to most people. If you search for a opacity/transparency icon, you often find something with two overlapping circles, however I think that would be too close to our path-intersection etc. icons.
I think this should be the last icon needed for https://bugs.kde.org/show_bug.cgi?id=408283, so I'm tagging this as closing that bug.