Assets can have filters

[Zulko]

Motivation:

Users may want to apply filters like "blur" or "black and white" to one asset of they project. This ticket is not to implement these effects already but more to put the basic architecture in place, possibly with one easy demo effect like black-and-white

Specs

• The Asset menu has a new tab "Filters". Where users can add filters using a "New filter" button.
• A dropdown menu enables to choose the filter type. Filter parameters will depend on the filter type.
• The architecture makes it easy to add new filters (in the gix-renderer, and in the online editor).
• This comes delivered with one demo effect like black-and-white.

Technical notes

• Filters will very probably be implemented at the FrameServer level
• JS libraries for image effects exist and can be an inspiration