Added licensing string constants to utils/strings

Added these three constants to utils/strings:
- INTERACTIVE_NOTICE
- INTERACTIVE_LICENSE
- INTERACTIVE_WARRANTY

These were based of the license text's notices for interactive programs,
found in the "How to Apply These Terms to Your New Programs" section.

Signed-off-by: Christina Sørensen <christina@cafkafk.com>

src/utils/strings.rs

@@ -13,3 +13,30 @@
 //
 //    You should have received a copy of the GNU General Public License
 //    along with this program.  If not, see https://www.gnu.org/gpl-3.0.html.
+
+/// Contains the notice for interactive programs from the GPLv3's "How to Apply
+/// These Terms to Your New Programs"
+const INTERACTIVE_NOTICE: &str = "\
+    gg  Copyright (C) 2023  Christina Sørensen <christina@cafkafk.com>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+";
+
+/// Contains the license part of the long notice for interactive programs from
+/// the GPLv3's "How to Apply These Terms to Your New Programs"
+const INTERACTIVE_LICENSE: &str = "\
+    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.
+";
+
+/// Contains the warranty part of the long notice for interactive programs from
+/// the GPLv3's "How to Apply These Terms to Your New Programs"
+const INTERACTIVE_WARRANTY: &str = "\
+    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.
+";