Drop InvisibleCollectionFetch in Favor of KDescendantsProxyModel

As the InvisibleCollectionFetch fetch strategy is currently broken, I
implemented a suggestion by Christian Mollekopf of working around the
issues by flatting the model through a KDescendantsProxyModel to fix
crashes in Plasma related to calendars.

BUG: 279322
FIXED-IN: 4.7.4

CCMAIL: chrigi_1@fastmail.fm
REVIEW: 103088