Replace kaccounts KCM with the one from plasma mobile

Summary: plasma-settings has a QML version of the accounts KCM. Since there's no point in using a separate KCM for dekstop and mobile use it on the desktop as well. Code is a almost direct copy from plasma-settings

Test Plan: Opened KCM. Added a google account

Reviewers: #plasma, bshah, dvratil, ognarb

Reviewed By: #plasma, bshah, ognarb

Subscribers: ognarb

Differential Revision: https://phabricator.kde.org/D24133