I originally started wanting to add a feature but it quickly started getting into refactoring mode with other issues that I saw. This is a high level list of what was changed

1. Renamed references of "rulerassistant tool" to just "assistant tool"
2. added ability to change opacity and color of all assistants
3. moved the painting logic to all be under the decoration. This makes it easier to organize when UI elements are displayed
4. assistant control widget is now displayed on top of the assistants
5. Changed the default assistant color
6. Made the assistant control widget active and delete buttons larger so they are easier to click.
7. The previews do not show when in the assistant tool...only when painting
8. fixed a bit of code with how the widgets mapped to the canvas (eg. vanishing point X when zooming)
9. Some spline tool fixes with how it is displayed and things were shown when they should have been hidden)
10. A lot of refactoring with moving code from the assistant tool to the decoration

There could be more refactoring...but I thought this might be a good place to stop for now.

You can apply the diff...but it might be easier to just check out my branch..

petrovic/assistants-opacity

1. Add different assistants to the canvas
2. change the color
3. change the opacity
4. toggle between the painting tool and the assistant tool
5. turn on and off the preview
6. turn the assistants on/off
7. delete the assistants
8. load/save assistants (even though I didn't touch this)