move all launcher config reading/saving into GroupManager

GroupManager can now have a config group provided by a subclass;
a siple set/get method pair wasn't enough as Applet::config() can
change without notice (principle: Applets should never hang on to
the results of config(), but call it, use it, then discard the value
upon returning from the method it is used in)

upsides:
* this is all in one neat, centralized place allowing launcher config
to be used amongst multiple implementations
* less code duplication just to satisfy config
* the launcher[Added|Removed] signals are now gone, which gets rid of the
somewhat ugly implementation detail that there were multiple ways to
find out when a launcher was added, some more direct than others

also, adds a configChanged signal which allows launchers to be saved
immediately to the config, safe(r) against interuptions / crashes