When used by QML, only populate after component is complete

Summary:
This introduces the use of QQmlParserStatus to delay populating
the model until all properties have been set, to avoid delegate
churn.

TasksModel is also meant to be used by C++. There's no good way
to determine whether an object is being instanciated by QML
during construction time, therefore this patch also introduces a
delay in initial population of the model after construction via
a single-shot timer. At the time the slot is invoked we know if
we're used by QML (because QQmlParserStatus::classBegin has
either been called or not by then) so we can decide to populate
or wait more for QQmlParserStatus::componentComplete.

I'm not super happy with this behavior change for C++ users,
however as the model is usually used via QML currently, it's
pragmatic to optimize performance for the common case, and it
doesn't technically break QAbstractItemModel semantics, as model
population isn't required to be sync.

There's a decent change this fixes a recently-reported crash as a
by-product:
CCBUG:386630

Reviewers: #plasma, davidedmundson, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8723