Call setDesktopFileName(...) to set proper app-id on wayland

[marcusbritanicus]

This PR fixes the app-id of this application.

Currently, the application name (screengrab) and organisation domain (https://lxqt.org) are set. Since desktopFileName is not set, Qt in a misguided attempt inverts the org-domain (giving us org.https://lxqt) and then appends the application name, to give us an app-id, resulting in this beautiful expression: org.https://lxqt.screengrab

This fix for this is simple: just call QApplication::setDesktopFileName(...).

[stefonarch]

Unfortunately it has no effect still:

$ lswt
app-id:                           title:
org.https://lxqt.screengrab       ScreenGrab

[stefonarch]

Forgot about screengrab always minimized to tray, long time ago I used it.
GTM