RFC: License change for skeleton template (split demo project)

[miurahr]

Because skeleton project include GPLv3 source, skeleton itself is GPLv3.
It may be restrict developers to produce a plugins that has different license.

We can split skeleton project to two individual project.

Thin skeleton: there is new skeleton without any OmegaT (GPLed) sources. licensed with permissive (MIT or BSD-3)
Filter demo: current skeleton project. licensed with GPLv3