Move time code from old-panel into an applet

Removes the last of `old-panel`.

old-panel/src/deref_cell.rs

@@ -1,31 +0,0 @@
-use once_cell::unsync::OnceCell;
-
-/// Wrapper around `OnceCell` implementing `Deref`, and thus also panicking
-/// when not set (or set twice).
-///
-/// To be used in place of `gtk::TemplateChild`, but without xml.
-pub struct DerefCell<T>(OnceCell<T>);
-
-impl<T> DerefCell<T> {
-    #[track_caller]
-    pub fn set(&self, value: T) {
-        if self.0.set(value).is_err() {
-            panic!("Initialized twice");
-        }
-    }
-}
-
-impl<T> Default for DerefCell<T> {
-    fn default() -> Self {
-        Self(OnceCell::default())
-    }
-}
-
-impl<T> std::ops::Deref for DerefCell<T> {
-    type Target = T;
-
-    #[track_caller]
-    fn deref(&self) -> &T {
-        self.0.get().unwrap()
-    }
-}