A lot of other painting applications have this and I thought it would be useful. This patch adds reference lines to the vanishing point assistant. It also adds a slider for people to change the angle increment for it. This slider is in the tool options.
I had to do a couple other things to make this possible.
1. Add the concept of a "selection" for assistants. The angle option will only appear in the tool options if a vanishing point assistant is selected
2. Added an ability to have assistant specific properties. No other assistant types have an angle slider for example
I also added the ability to load/save from a KRA....and load/save from a painting assistant file with this new custom property
These lines are part of the "preview", so turning off the assistant preview will hide these lines if people don't want to see them.