[ADD] slides gallery module

- Task: https://www.odoo.com/web?#id=9093&view_type=form&model=project.task
- Pad: https://pad.odoo.com/p/r.QlQTvBv6mCcMrK4B
If I understand well your code, categories are simply subdivisions of channels (and use duplicated code). 
@tde-banana-odoo I think we should avoid 2 models that do the same job... can I have your opinion on that?