GlobalMenu: Fix opening About Page on Global Menu

I could not find an associated bug report, I noticed this issue myself. 🙂

This MR fixes the About Page link on the Global Menu by bringing it up to speed with the new logic to open the About Page introduced with !277 (merged).

The "About System Monitor" link in the Global Menu broke again. It was previously fixed in !272 (merged), where the issue was that the path to the About Page was slightly incorrect. However now the logic to open the About page has changed again, and the Global Menu section was not updated. It now uses the same Qt.createComponent that !277 (merged) uses.

Without this change, the About Page cannot be opened from the Global Menu, simply nothing happens. But some pretty funky things happen if you try and then try to open the About Page from the hamburger menu and press the "Back" arrow. I believe since System Monitor doesn't know which QML page to go back to, it does some strange UI behaviour where it tries to close the About Page but cannot really go back to anything. It's pretty weird, and that is fixed as a result of this MR as well.😀

Please let me know if I did anything wrong, I'm not too experienced with KDE development and pretty much mirrored the change seen in Main.qml, and confirmed that it fixes the issue.