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/dccgui.c

@@ -192,7 +192,7 @@ dcc_store_color (GtkListStore *store, GtkTreeIter *iter, int column, int color_i
 	if (color_index != 1)
 		color = &colors[color_index];
 
-#if GTK_CHECK_VERSION(3,0,0)
+#if HAVE_GTK3
 	if (color)
 	{
 		GdkRGBA rgba = *color;