[preferences: add option to remove/hide brush outline]

I like to make screen recording videos of me creating drawings and paintings. To do this, I chose not to use Affinity Photo (even though I otherwise kind of like the program) and use a competitor that allows me to turn off the cursor/brush preview. Having the cursor/brush on during these videos takes away the "magic quality" when making these videos. Please add the ability in preferences to do this. Thank you.