Check values have changed before setting the model

Summary:
Only save data that needs to be saved and use sanitized data rather than raw text field input.

BUG: 384894
FIXED-IN: 5.18

Test Plan:
Edit Username -> have a PolicyKit password verification
Edit Avatar -> no more password verification popup

Reviewers: #plasma, ngraham, davidedmundson

Reviewed By: #plasma, ngraham, davidedmundson

Subscribers: ngraham, apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25398