preserve the current clipboard when using the Clipboard backend.

Store the current clipboard and restore it after the expansion has been
pasted.
This prevents your current clipboard from being lost if using the
Clipboard backend.

src/engine.rs

@@ -205,8 +205,10 @@ impl <'a, S: KeyboardManager, C: ClipboardManager, M: ConfigManager<'a>, U: UIMa
                         }
                     },
                     BackendType::Clipboard => {
+                        let previous_clipboard_content = self.clipboard_manager.get_clipboard().unwrap_or(String::from(""));
                         self.clipboard_manager.set_clipboard(&target_string);
                         self.keyboard_manager.trigger_paste(&config.paste_shortcut);
+                        self.clipboard_manager.set_clipboard(&previous_clipboard_content);
                     },
                 }