Revert "Fix `drawCircle` signature in `PaintingWindow`."

This reverts commit bfc4b49dff33c48682b8c8d97afde39ef7db6758.
Revert "Change signature of painting commands, update doc, hide functions using Qt objects."

This reverts commit d14cc4c2cd5228d7d99477833a36829a4885931a.
Revert "Fix wrong variable name."

This reverts commit aebeb0b4fe2191a2e54d64adfb1138e4103d09be.
Revert "Remove obsolete painting function."

This reverts commit bde57b76411e9cbbbada54ab9394339050e20d75.