[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 this variable will always contain a string, you can put `""` (the empty string) as init value, so that we already know it will contain a string (you wouldn't put `False` if Python was statically typed, wouldn't you?)