Improving effect management

Switch to white list instead of black list

• Switch to white list (ME: would allow to check with tests if all effects are available, #1850 (closed))
• Allow custom white lists
• Allow disabling all white lists for testing / advanced users

Add a link to the documentation for each effect

• Add link to online documentation in effects / compositions info

For instance : https://docs.kdenlive.org/fr/effects_and_compositions/video_effects/alpha_mask_keying/rotoscoping.html