Gumpu/make assistant guidelines follow brush during stroke

The request contains two changes:

1. It makes the guidelines for the parallel ruler painting assistant follow the brush position instead of the cursor position during a store with the free hand tool.
2. It adds unit tests for the parallel ruler assistant and the PaintingAssistantsDecoration (in libs/ui/tests)

This is a partial fix for Bug 415208.

If this solution is deemed acceptable I can implemented it for the other painting assistants and add more unit tests.

Test Plan

• Create a new document
• And one or more parallel rulers
• select the freehand tool
• start a stroke
• after a while move in random directions.

Result

The guidelines follow the brush position instead of the cursor position.

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.