style: add missing license headers

The MPL is a file-based license, rather than a codebase-based license.

Each files need to be marked as MPL.

If we don't do mark them, they may cause confusion.

src/components/window/file.rs

@@ -1,3 +1,6 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use glib::{
     clone

src/components/window/imp.rs

@@ -1,4 +1,7 @@
-
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+ 
 use gtk::prelude::*;
 use gtk::subclass::prelude::*;
 use gtk::CompositeTemplate;

src/components/window/menubar.rs

@@ -1,3 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
 use super::file::FileImplementedEditor;
 use super::EchidnaWindow;
 use gio::{MenuModel, SimpleAction};

src/components/window/mod.rs

@@ -1,3 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
 mod file;
 
 mod imp;

src/components/window/sidebar.rs

@@ -1,3 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
 trait SidebarImplementedEditor {
     fn setup_sidebar();
 }

src/components/window/window.ui

@@ -1,4 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this
+   - file, You can obtain one at https://mozilla.org/MPL/2.0/. -->
 <interface>
   <requires lib="gtk" version="4.0" />
   <template class="EchidnaWindow">