Setup QQmlEngine only once in QmlObject

setupBindings is called for every QmlObject created and installs various bits on the QQmlEngine, such as a network access manager,
a theme icon provider, etc. However, since we use a shared QQmlEngine in various places, most of this needs to be done only once.

This splits setupBindings into a setupContext method for setting up the localized context needed for every instance and a setupEngine
static function that setups an engine. A fourth protected constructor is added that allows to influence the setup behavior.
While at it, delegating constructors are used still calling both setup methods to not change behavior.

Differential Revision: https://phabricator.kde.org/D9141