window: Don't store window to avoid ref cycles

Use root() instead and move one constructor to on root for having the
root available.

src/util/root.rs

@@ -0,0 +1,34 @@
+// Copyright (c) 2024 Sophie Herold
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+use gtk::prelude::*;
+
+use crate::widgets::LpWindow;
+
+pub trait ParentWindow: WidgetExt {
+    fn window(&self) -> LpWindow {
+        let result = self.root().and_downcast();
+
+        if result.is_none() {
+            log::error!("Couldn't find LpWindow for {self:?}");
+        }
+
+        result.unwrap()
+    }
+}
+
+impl<T: WidgetExt> ParentWindow for T {}