shell installed: Simplify the code for adding pending apps

Previously we used a magical timestamp as a guard to avoid listing
pending apps more than once. Instead, we'll now check if the app is
already listed before adding it, hopefully improving code readability.
Improve the sorting order on the Installed page

Previously, the sorting algorithm would take into account the install
timestamp, which happened to be 0 for most apps. But not all, so some
apps would seemingly randomly move to the bottom of the list after
reinstalling (and a timestamp > 0 through that).

Fix this by simply sorting by name.

Also, while at this, make sure the sorting algorithm treats upper and
lower case letters the same.
Show system apps on the Installed page

This lets people check what apps are installed as part of the system,
check their version, and move them to different folders if needed.

I've added a new "System Applications" header which should hopefully be
enough to explain the lack of the Remove button.
Updated Turkish translation