Updated the chanview API to use GTK3-safe font descriptions while keeping GTK2 style handling conditional in the header and implementation.

Guarded tree view palette application to use GTK3 font descriptions or GTK2 styles appropriately.
Adjusted the chanview creation call site to pass GTK3 font descriptions or GTK2 styles based on the build.

src/fe-gtk/chanview-tree.c

@@ -116,11 +116,17 @@ cv_tree_init (chanview *cv)
 
 	view = gtk_tree_view_new_with_model (GTK_TREE_MODEL (cv->store));
 	gtk_widget_set_name (view, "zoitechat-tree");
-	if (cv->style)
+	if (
+#if HAVE_GTK3
+		cv->font_desc
+#else
+		cv->style
+#endif
+	)
 	{
 #if HAVE_GTK3
 		gtkutil_apply_palette (view, &colors[COL_BG], &colors[COL_FG],
-		                       cv->style->font_desc);
+		                       cv->font_desc);
 #else
 		gtkutil_apply_palette (view, &cv->style->base[GTK_STATE_NORMAL],
 		                       &cv->style->text[GTK_STATE_NORMAL],