Make the default label colour white (#282)

We should be following the system theme here, probably.

And doing a lot of other things. But this is a saner default with a
black default "background"

crates/xilem_masonry/src/view/label.rs

@@ -8,7 +8,7 @@ use crate::{Color, MasonryView, MessageResult, TextAlignment, ViewCx, ViewId};
 pub fn label(label: impl Into<ArcStr>) -> Label {
     Label {
         label: label.into(),
-        text_color: Color::BLACK,
+        text_color: Color::WHITE,
         alignment: TextAlignment::default(),
         disabled: false,
     }