qmllint: warn when shadowing signals and handlers

See attached project: qmllint does not warn about shadowing signal, signal handlers, property changed signals and property changed handlers with properties, methods or signals.

Shadowing those special names is bad because it makes writing tooling (especially: qmlls) more complicated, it has weird behaviors in the engine and some of this shadowing is also not supported by the engine.

Uncomment each line separately to see the engine's error. You can also uncomment them all (but the engine aborts after the first error) to see that qmllint does not complain at all.