Add DW_FEATURE_TREE feature test, since iOS does not suppor tree. Update dwtest to show a placeholder when missing the tree widget. Remove DW_FEATURE_CONTAINER_STRIPE from GTK4. Apparently it is supported with the GtkListBox widget and custom CSS, but it performs badly and is unrecommended by the GTK development team. Finally add missing feature test example code to the template source.

# Set these here to change the version for all platforms
DW_MAJOR_VERSION=3
DW_MINOR_VERSION=2