Set the desktop filename of the Qt application

Currently, the app ID of the Dangerzone GUI application when running
under Wayland is `python3`, which is not very useful if one wants to
automate some action related to the Dangerzone application window (e.g.
to always start Dangerzone window in floating mode under Sway WM).

Setting the desktop filename property also sets the app ID of the
application under Wayland. According to Qt documentation[1], the
property value should be the name of the application's .desktop file but
without the extension.

Qt documentation also states:

> This property gives a precise indication of what desktop entry
> represents the application and it is needed by the windowing system to
> retrieve such information without resorting to imprecise heuristics.

Therefore I also think that setting this property is needed to display
the correct application name and icon (taken from the .desktop entry)
when running under certain windowing systems (like Wayland)
(see also #402).

[1]: https://doc.qt.io/qt-6/qguiapplication.html#desktopFileName-prop

dangerzone/gui/__init__.py

@@ -122,6 +122,13 @@ def gui_main(dummy_conversion: bool, filenames: Optional[List[str]]) -> bool:
     # Create the Qt app
     app = Application()
 
+    # Needed under certain windowing systems to match the application to the
+    # desktop entry in order to display the correct application name and icon
+    # and to allow identifying windows that belong to the application (e.g.
+    # under Wayland it sets the correct app ID). The value is the name of the
+    # Dangerzone .desktop file.
+    app.setDesktopFileName("press.freedom.dangerzone")
+
     # Common objects
     if getattr(sys, "dangerzone_dev", False) and dummy_conversion:
         dummy = Dummy()