[Merged] Masking panel revamp and new masking possibilities

[Masterpiga (Daniele Pighin)]

I added updated doc pointers here:

[New masking panel] [Flexi] Interim user docs (subject to change)

After the new masking panel has been pushed to master, a lot of users who had not followed the development found themselves in the difficult position of having to get used to a new masking system (both the model and the UI) without any kind of documentation other than tooltips (when enabled!). While full documentation will be released only around the time that 5.8 is ready, so that it can include concrete references to the UI that we will sheeip ( vs. ) , I wanted to share some ear…

@Eddie_Lambrechts @Fenny @priort @Underexposed @perriko @bazdesh @don51 as you all had some questions more or less recently.