LibDesktop+Taskbar: Add 'WorkingDirectory' property to app files

Author: https://github.com/cflip
Commit: 05e7b338ad
Pull-request: https://github.com/SerenityOS/serenity/pull/15646
Reviewed-by: https://github.com/gmta

Userland/Libraries/LibDesktop/AppFile.h

@@ -29,6 +29,7 @@ public:
     String executable() const;
     String category() const;
     String description() const;
+    String working_directory() const;
     String icon_path() const;
     GUI::Icon icon() const;
     bool run_in_terminal() const;