Add the Continuous Funding Category and LibreSelery

[Ly0n]

Added the "Continuous Donation" concept that we created in the protontypes organisation with LibreSelery. The concept was already tested for developing LibreSelery itself. I invested around 2000€ in the development and distributed the donation among the contributors of the main project and some contributors of our dependencies.

It would also be nice to get some feedback on this new concept.