qml interop: start replacing QMetaObject::invokeMethod with signals

It seems concurrent calls to invokeMethod using qml component
object pointers can lead to access violations. These method
invocations can be replaced with a signal/slot mechanism.

This patch replaces only the invocations in conversationsadapter
and accountsadapter that are known to be problematic for now.

Some code cleanup is done for QmlAdapterBase derived classes.

Gitlab: #61
Change-Id: I72f47c9229a9bf42299ae52822c3a1a8c04eb423