focus-stack: Keep dragged surface in focus stack in move_request()

Replacement for https://github.com/pop-os/cosmic-comp/pull/1687, that
works correctly with multiple outputs.

We don't want another window to show a focus indicator while a window is
being dragged, so keep the window in the focus stack. If a window is
being moved out of a stack, change the focus from the stack to the
window.

`refresh_focus_stack()` doesn't seem to be called here, but for good
measure, make sure that calling that function also won't remove a
`CosmicMapped` from the focus stack if it is currently part of a move
grab for the seat.

src/shell/focus/mod.rs

@@ -129,11 +129,11 @@ impl FocusStackMut<'_> {
         self.0.insert(target);
     }
 
-    pub fn remove<T>(&mut self, target: &T)
+    pub fn remove<T>(&mut self, target: &T) -> bool
     where
         T: Hash + indexmap::Equivalent<FocusTarget>,
     {
-        self.0.shift_remove(target);
+        self.0.shift_remove(target)
     }
 
     pub fn last(&self) -> Option<&FocusTarget> {