[Feature Request] Symmetry

[prostolyubo]

I primarily use Piskel and one of the features I miss here is a symmetry toggle.

The simplest implementation could add toggles for horizontal and vertical symmetry. The better implementation could also add option to move the symmetry lines (including placing a line in between pixels to allow symmetry on images both with odd and even number of pixels).

To clarify: symmetry should apply any changes to pixels on one side of the symmetry line to the other. A simplified approach would be to just add symmetry option to a pencil/brush.