Allow launching panel subpages
This allows us to open subpages of the "System" panel. The arguments/ parameters are expected to match the panel's subpage tags (as in AdwNavigationPage.tag).
With this, a future change could reintroduce desktop files for the subpages, and just change the Exec line to accommodate the new format.
If this way to go about this problem is accepted, I will update this MR generalizing it to other hub panels such as "Privacy".