QtCurve/Qt5 : further KF5 adaptation

Introduces several improvements to QtCurve's integration with KF5:

 * Inherit KStyle instead of QCommonStyle when built with KDE support
 * recognise at least KF5 systemsettings application and esp. KWin5
 * use the new method of being informed about changes to the
   compositing mode
 * detect when KWin5 adapted its colour palette to an application's
   dedicated palette.

REVIEW: https://phabricator.kde.org/D5178