ActionButton icon coloring

Subsurface uses themes in a way that is very different from the Kirigami vision. Kirigami appears to want the app to always follow the OS theme - which is logical if you think about Kirigami as part of the larger Plasma framework and the goal is all about consistency. But if one uses Kirigami for a one off application that will run in a number of different environments without any other Plasma apps, that is less appealing. Instead, we have opted for having our own color schemes that the user can switch between. And that constantly bumps into issues with the way Kirigami uses its themes. At some point I would like a larger conversation about THAT, but for now I'll submit this MR that fixes what I believe to be a bug in current Kirigami:

The ActionButton code assumes that there is a property 'color' on a Kirigami.Action and then uses that to override the color for the icon. Unfortunately that property didn't exist - and for strange reason all three buttons were implemented using the color of the center button, which is kinda weird since they have different backgrounds.

This commit fixes both of those things. If the Kirigami designers would prefer to keep things to always use the color set on the center button (I couldn't imagine why) then I'm happy to split this into two commits one of which you could ignore. Just let me know.