main/postmarketos-ui-gnome: add two useful apps to recommends
These are useful on desktops/laptops/tablets.
This was only added to the gnome (desktop) UI because:
-
The system monitor could be useful on mobile devices however the UI is not adaptive.
-
Disk utility isn't that useful on mobile devices since it tends to show dozens of partitions on android phones, and so it's kinda confusing and it would be easy for users to mess things up.