Tooling · Workbench: a playground, and the gallery as the workbench

tooling::Playground lays a preview beside its knobs, each a switch, a
choice or a number that edits one property; the preview redraws from
the settings, which fail on a name or kind the knobs lack. The gallery
app is the ComponentGallery; its page 43 shows both.

src/tooling/mod.rs

@@ -0,0 +1,5 @@
+mod playground;
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
+
+pub use playground::{Knob, Playground, Setting, Settings};