New gradient fill features for All apps

[MxHeppa]

- fill where i can select colours four corners and get in beetween colours middle of them.
- other way mix colours in gradient where this mode uses HSL colour mode in beetween colours not RGB mode. this way when end colours are pure red and green middle has yellow etc. one bitmap tool has this but has so many  limtis (no true colour but many platform speffic things) and is about 25years old. i still going use it some other things but i deream this also modern Affinity Pgorams. is Photo side becuse i like most of these for creative graphics when i dont need vector stuff.