Fix colours not updating in systemsettings

Summary:
(after a lot of looking, it turns it's out not a bug in the QQC desktop
style)

KSharedConfig::openConfig() opens the config based on the app name.

The app name comes from the exec name, or
KAboutData::setApplicationData.
setApplicationData only works on an existing qApp.

This means the QPT loaded in the QApp uses the wrong config.
The effect is minimal because most stuff is in kdeglobals, but it means
the KSharedConfigPtr is not shared with the one used elsewhere in the
app. This is wasteful, but also means the QPT calling
reparseConfiguration doesn't take effect, leading to colours not being
dynamically updated

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8399