Do not ask for root permissions when it's unnecessary

Summary:
Only save values that were really changed

This allows to change settings without being asked for the root password
to save things that weren't modified by the user, so the
request for the root password was unnecessary.

Remove unneeded UncacheUser/CacheUser calls

Modifying the Real Name of the user was requesting the root password
because of the calls to UncacheUser/CacheUser, since modifying the Real
Name doesn't in fact need extra permissions. Also, none of the other
properties call UncacheUser/CacheUser when they're being modified, so
I tried removing the calls and it works much better here.

Test Plan:
I tested changing avatar and Real Name and looking at /etc/passwd and ~/.face
that they were correctly changed while not getting any polkit dialog
requesting the root password.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3102