[Tails 4] Regression: Desktop icons no longer show startup notification

Tails 3.x shows a startup notification (the mouse cursor turns into a spinner) when you launch an application via its desktop shortcuts (e.g., the “Report an Error” icon). This is accomplished via the `StartupNotify=true` setting in the shortcut, set in the `.desktop` file for a given shortcut.

This type of user feedback is important, as applications can take a long time to launch from a USB drive.

Unfortunately, in Tails 4 (tested in beta1 and RC1), these notifications no longer appear. To reproduce, double-click any icon on a Tails 4 desktop, and note the lack of any obvious notification to the user that an application has been launched.

Original created by @eloquence on 17160 (Redmine)