* src/misc/configuration.c, include/configuration.h: added a change_action_add()method to associate an action (that can be triggered through a gui) to a config var.
* modules/gui/wxwindows/preferences_widgets.*: implemented partial support for change_action_add().
* modules/access/dshow/dshow.cpp: use change_action_add() to refresh the list of devices and to add an option to configure a device.