gtkcolorbutton: Invalidate the style context on select

Forcing the GtkDialog style context into an invalid state prevents a bug
under the Wayland backend whereby double clicking on a color swatch
prevents the color picker dialog from re-appearing. (note: this patch is
not a bug fix, just a demo of a workaround).

https://bugzilla.gnome.org/show_bug.cgi?id=695493