Symmetry lines #7

Currently there is no way for the user to see if Symmetry is enabled and what kind of symmetry.

This can be shown with lines, indicating where the symmetry happens.

When the user have selected the "3 way symmetry mode", then there are 3 lines dividing the canvas.

When the user have selected the "xy symmetry mode", then there are 1 horizontal line and 1 vertical line dividing the canvas.

The user can enabled/disable these lines in preferences. Maybe the lines should auto hide/show while the user is drawing.