gtkcolorbutton: Invalidate the style context on color select

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

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