[kded] Save replication source hash to config file

Summary:
When a replication source is set write its output hash to the config file and
load the value again on next startup. If no replication source is set the
value written is an empty string.

Test Plan: Tested on X11.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23733