Use natural sorting in ActivityModel

Summary:
ActivityModel sorts internally.
It currently does a non-locale or number aware sort on user-facing
strings.

This patches uses QCollator.

This changes the order in the KCM.

It doesn't affect the order in the activity switcher as that has it's
own sorting proxy on top handling last used time.

It doesn't quite fix the issue in the bug report as PagerModel mixes
sources.

CCBUG: 404149

Test Plan:
Set activities named
"10.asdf"
"1.foo"
"2.bar"

KCM now shows them in the order 1,2,10

Reviewers: Plasma

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18957