Make SolveApp::start honour a command's enabled flag, and disable solid.move inside a sketch (#740)

`SolveApp::start` now honours a command's enabled flag, so shortcuts, the command palette and the control channel can't start a disabled command, and Move is disabled while a sketch is open.

crates/engine/src/cmd/mod.rs

@@ -186,6 +186,12 @@ pub fn in_sketch(s: &Session) -> std::result::Result<(), String> {
     if s.active_sketch.is_some() { Ok(()) } else { Err("no sketch is being edited (Create Sketch or Edit Sketch first)".into()) }
 }
 
+/// The mirror of [`in_sketch`], for a command that acts on bodies and has nothing to offer the
+/// geometry under the cursor while a sketch is open.
+pub fn not_in_sketch(s: &Session) -> std::result::Result<(), String> {
+    if s.active_sketch.is_none() { Ok(()) } else { Err("a sketch is being edited (this works on bodies: Finish Sketch first)".into()) }
+}
+
 /// Every command, in toolbar order.
 pub fn command_specs() -> Vec<&'static CommandSpec> {
     let mut v: Vec<&'static CommandSpec> = Vec::new();