feat(quickshell): close the window on a middle click of its title

Closing goes through the window id rather than niri's focused-window default:
the click and the moment niri acts on it are not the same instant, and the
focus can move in between.

defaults/xdg/quickshell/moonarch/Niri.qml

@@ -131,4 +131,18 @@ Singleton {
     Process {
         id: focusProc
     }
+
+    // Close a window by id rather than by focus: the click that asks for this
+    // and the moment niri acts on it are not the same instant, and the focus
+    // can move in between.
+    function closeWindow(id) {
+        if (id === null || id === undefined)
+            return;
+        closeProc.command = ["niri", "msg", "action", "close-window", "--id", String(id)];
+        closeProc.running = true;
+    }
+
+    Process {
+        id: closeProc
+    }
 }

defaults/xdg/quickshell/moonarch/WindowTitle.qml

@@ -64,6 +64,13 @@ Row {
     HoverHandler {
         id: hover
     }
+
+    // Middle click closes the window the title belongs to.
+    TapHandler {
+        acceptedButtons: Qt.MiddleButton
+        enabled: root.win !== null
+        onTapped: Niri.closeWindow(root.win.id)
+    }
     Tooltip {
         anchorItem: root
         shown: hover.hovered