Improve the color picker plugin

• The regex color matching is simplified
• Defaults are tweaked (do not match named colors and translucent colors)
• Fixed color note updating (e.g. for search and replace)

This MR does not address, e.g. the proper handling of transparent colors and the per-file type configuration

Sorry about the barrage of commits. I kept the "outdated" commits (e.g. jbatrina/kate@f85b75fd) to preserve development history, but I'll remove them if needed.

(Also I'm not sure who should be the assignee, but since @cullmann said he doesn't use this plugin, I assigned to @dhaumann instead)