qmlplugindump ignores composite types

Creator as shipped with Qt 5.1.0 does not do syntax completion for the usual dialogs in the QtQuick.Dialogs module. src/imports/dialogs/plugins.qmltypes does not mention them.

My theory as to why:

When a C++ plugin registers a QML file as a new type, it's a composite type. qmlplugindump ignores composite types (and contains a TODO to implement dumping later). So we need to make that work in order to provide a smoother development experience. (Or else we have to write those parts of the qmltypes files by hand, but that doesn't seem worthwhile.)