Avoid QQmlCustomParser in Qt code

QQmlCustomParser wants to process the QML source code at runtime. This is a problem when compiling QML to C++. Most custom parsers also introduce concepts to the language that don't quite fit. We should find other ways to provide the functionality.

Elements with custom parsers we have addressed already:

• Connections has a late bound mode that lets you declare handlers as regular js functions. It warns about usage of custom parsed signal handlers.
• PropertyChanges uses generalized grouped properties now (but doesn't warn about custom parsed ones yet).

Remaining pain points:

• ListElement. We should just replace ListModel with list<Foo> where Foo has to be a specific type.
• others?