revisiting the sycoca

[David Edmundson]

> All in all I am not convinced we still need the sycoca

As you noted it's 3 different things. I suggest we tackle it as 3
different tasks.
The mime apps part we should be able to drop silently without any issue.

The menu and desktop files are still needed by krunner and plasmashell
and xdg-desktop-portal.
I don't think an external cache is problematic here, it works. It
could be done differently, but we're just moving the problem about and
I don't think there's any immediate savings here.

What we should be looking at as a first step is splitting "what
applications need" from "what do system services need" and making sure
these are separate and covering the right things.
Apps seem to mostly use serviceByDesktopName that doesn't need to be
backed by a cache, at worst we have to stat 4 or 5 folders.  I propose
we replace that with a lighter desktop file wrapper higher up in the
stack (Kconfig?)  and port those instances in apps.  Or maybe it
should be backed by some relevant portal call.

David