Bug 318347 grid brush keeps grid alignment when increasing brush size

This MR addresses a wishlist item on bugs.kde.org to keep the grid alignment on the grid brush when increasing the brush size. This introduces two new brush settings "Cell width" and "Cell height" while removing the "scale" setting. This is to set exactly what the size of each cell in the grid is instead of it being determined by the size of the brush.

Test Plan

1. Select a grid brush and draw a stroke
2. Increase brush size
3. Draw a new stroke. The ellipses in both strokes should keep alignment and be the same size

This test should be repeated with varying brush sizes and cell sizes.

Questions

I am unsure if the "Grid width" and "Grid height" should still be kept in the brush editor. Now that the brush size no longer alters the functionality of the brush I'm not sure if there is some other need that would make it useful there. What do you all think?

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.