[GTK3] Recolor CSD titlebuttons according to user's color scheme
Window titlebuttons on CSD windows now respect the user's colorscheme, preventing colorschemes such as Breeze Light, which has black on white instead of white on dark window buttons, from having barely discernable window buttons.
Chrome/ium has been exempted from this due to bugs from the Chromium side of things.
Subscribers: ognarb, plasma-devel
Differential Revision: https://phabricator.kde.org/D23451