Introduce messageserver plugins.

MessageServer plugins will be loaded and executed in the messageserver main loop,
these plugins should only be used for services that need to know information
about the operations initiated by all clients connected to messageserver.

To make use of this functionality qmf should be compiled with 'DEFINES+=MESSAGESERVER_PLUGINS'.

Change-Id: I6bebb150645d2c70a5bc67ffcaaa904f94ed3705
Signed-off-by: Valerio Valerio <valerio.valerio@jollamobile.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>