Switched palette type selection and color conversion helpers to use the HAVE_GTK3 guard consistently in palette definitions and helpers.

Updated GTK3-specific palette color storage handling in DCC, notify, and user list views to align with HAVE_GTK3 guards.
Standardized setup color application and dialog logic to use HAVE_GTK3 branches for GTK3/GTK2 separation.

src/fe-gtk/userlistgui.c

@@ -473,7 +473,7 @@ userlist_store_color (GtkListStore *store, GtkTreeIter *iter, int color_index)
 {
 	const PaletteColor *color = color_index ? &colors[color_index] : NULL;
 
-#if GTK_CHECK_VERSION(3,0,0)
+#if HAVE_GTK3
 	if (color)
 	{
 		GdkRGBA rgba = *color;