Loader needs setSource counterpart for sourceComponent with initial properties

Background

See https://bugreports.qt.io/browse/QTBUG-125071 and https://bugreports.qt.io/browse/QTBUG-125070 on why this might be important in certain scenarios.

Problem

Loader QML Type implements a source & sourceComponent pair of properties, and a setSource() method. The latter allows assigning a new source URL with a set of initial properties. However, sourceComponent doesn't have such counterpart, making the Loader type inqonsistent with itself, and without it you can't even implement workarounds like described at the bottom of https://bugreports.qt.io/browse/QTBUG-125071

Solution

Introduce a setSourceComponent(Component sourceComponent, object properties) invokable method.