GTK3: Fix DW_CLR_DEFAULT support and fix a memory leak. During testing with the new dwtest code for Rich Edit MLEs on Windows... I discovered that the CSS color and font overrides in GTK3 were not being removed, causing DW_CLR_DEFAULT to not function after previous calls to dw_window_set_color() with other colors. We now save the CSS providers for the overrides and remove them on the next call. Previously we just added new CSS providers on each call leaking memory.