delay node initialization for python plugins until start of plugins (#67)

@ablasdel @DorianScholz What is your opinion on this? Is that better that how rqt currently behaves?

I could imagine one related feature: add a modal message box when a plugin start fails to make the message more visible. But that would require to collect the messages of multiple plugins when they are started together (either at start up or via a perspective) to not pop up numerous dialogs.