Make Breeze Dark and Light plasma themes follow system accent color

This patch removes the accent color from the colorscheme files of dark/light colorschemes; by doing this, we will always be following the accent color set by the system. The user can optionally change the accent color from "Custom" to "From color scheme", and this ensures that the accent color of Plasma will follow the accent color of the selected colorscheme, even if it's not Breeze.

This required the theme code to always reparse its configuration on color changes, in order to catch "upstream" default changes.

To test this, switch to e.g. Breeze Dark and try changing the accent color around.