Make Line Tool snap to assistants

I made Line Tool snap to assistants.

Test Plan

Test 1.

1. Create a new document for every assistant type.
2. Create a paint layer and a vector layer.
3. Use Line Tool (on both paint and vector layers) to create a few lines. Move the cursor around to see how the preview works and if the line ends up how you wanted.

Test 2.

1. Create a document with multiple assistant types.
2. See that they work together nicely.

NOTE: Spline, Fish Eye and Perspective Ellipse assistants shouldn't affect the Line Tool at all. For Ellipse and Concentric Ellipse I added a functionality to draw lines between two points on the ellipse.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.