Implement top-left hot corner to toggle the Overview

Compared to third-party implementations such as waycorner:

- It works during interactive window move (no surfaces receive pointer
  focus in this case, so this cannot work through layer-shell).
- It works during drag-and-drop.
- It disables itself over fullscreen windows.
- It does not prevent direct scanout.

niri-config/src/lib.rs

@@ -1210,6 +1210,8 @@ pub struct Gestures {
     pub dnd_edge_view_scroll: DndEdgeViewScroll,
     #[knuffel(child, default)]
     pub dnd_edge_workspace_switch: DndEdgeWorkspaceSwitch,
+    #[knuffel(child, default)]
+    pub hot_corners: HotCorners,
 }
 
 #[derive(knuffel::Decode, Debug, Clone, Copy, PartialEq)]
@@ -1252,6 +1254,12 @@ impl Default for DndEdgeWorkspaceSwitch {
     }
 }
 
+#[derive(knuffel::Decode, Debug, Default, Clone, Copy, PartialEq)]
+pub struct HotCorners {
+    #[knuffel(child)]
+    pub off: bool,
+}
+
 #[derive(knuffel::Decode, Debug, Clone, Copy, PartialEq)]
 pub struct Overview {
     #[knuffel(child, unwrap(argument), default = Self::default().zoom)]
@@ -4587,6 +4595,9 @@ mod tests {
                         1500.0,
                     ),
                 },
+                hot_corners: HotCorners {
+                    off: false,
+                },
             },
             overview: Overview {
                 zoom: FloatOrInt(