niri: add window-rule management

- settings UI for creating, editing, deleting window ruels
- IPC to create a window rule for the currently focused toplevel

fixes #1292

quickshell/Services/PopoutService.qml

@@ -33,6 +33,7 @@ Singleton {
     property var polkitAuthModalLoader: null
     property var bluetoothPairingModal: null
     property var networkInfoModal: null
+    property var windowRuleModalLoader: null
 
     property var notepadSlideouts: []