Fix Workspace::minimize for tiled fullscreen applications

Using `is_tiled()` here doesn't work for fullscreen windows, since
`is_tiled()` returns `false` for them.

This (plus the previous changes for supporting minimize) seems to fix
the behavior of `SDL_MINIMIZE_ON_FOCUS_LOSS` in XWayland applications
(https://github.com/pop-os/cosmic-comp/issues/231). Wine had a similar
issue, though I haven't tested it yet.

It doesn't seem ideal visually that the fullscreen window becomes tiled
then minimized itself, but that's less of a problem.

May need to check if this is an issue with any other uses of `is_tiled()`
or `is_floating()`.

src/shell/workspace.rs

@@ -585,7 +585,7 @@ impl Workspace {
             None
         };
 
-        if self.is_tiled(elem) {
+        if self.tiling_layer.mapped().any(|(m, _)| m == elem) {
             let was_maximized = self.floating_layer.unmap(&elem);
             let tiling_state = self.tiling_layer.unmap_minimize(elem, to);
             Some(MinimizedWindow {