Rename window -> tile, fix popup unconstraining window_y

I missed the window_y thing and found it thanks to the rename. Too lazy
to split it into a commit.

src/layout/tile.rs

@@ -91,7 +91,7 @@ impl<W: LayoutElement> Tile<W> {
     }
 
     /// Returns the location of the window's visual geometry within this Tile.
-    fn window_loc(&self) -> Point<i32, Logical> {
+    pub fn window_loc(&self) -> Point<i32, Logical> {
         let mut loc = Point::from((0, 0));
 
         // In fullscreen, center the window in the given size.