This was the common base class of IProjectPanelFactory and
ITargetPanelFactory. Nothing was using the IPanelFactory interface, and
there's actually no common interface between those classes.

Of the old interface IPanelFactory:
id() => only used in ITargetPanelFactory
displayName() => only used in IProjectPanelFactory
priority() => only used in IProjectPanelFactory

This removes lots of boiler plate code for the unused functions.

Change-Id: I8488a4e5134fd451907f02c45b7847673e4dc714
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>