qmljs: avoid ecessive updating when cxx types might have changed

perform an update of the qml code model only if a cpp file exporting
qml types has not changed for at least 15 seconds.

Change-Id: I674eed8dd5a68f2d74a8794fd8a237c2d99a3185
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>