get rid of blockSignals

Summary:
Don't call blockSignals, but instead, to avoid an infinite signal chain,
check KCoreConfigSkeleton::configChanged wasn't emitted by ourselves writing on the
config.
Plasma::Applet::configChanged() slot was never invoked when config was written from
QML, which is wrong.

Test Plan:
Config still loads, plasma which makes heavy use of this still loads and saves
configs correctly, no infinite signal/slot recursions.
Now if somebody connects to KCoreConfigSkeleton::configChanged from outside of
this class, it wil lreceive correct signals when somebody writes to the config from
QML.

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22250