make PlasmoidProtocol::loadFromConfig behave properly in all situations

this is done by greatly simplifying PlasmoidProtocol::loadFromConfig and putting a check
in PlasmoidProtocol::addApplet to see if the host already contains the applet. this is needed
anyways since addApplet can be (and is) called from the manager.

this also adds a check such that if for whatever reason more than one instance of the same
applet was present in this system tray (something which the current design expressly prohibits
though i do wonder if it doesn't make sense to have more than one, e.g., weather widget at a time)
then delete the stray config group, preventing endless #s of configs for the same applet
piling up in the config should anything have gone wrong in the past