Option to flip to other side of plane

[K-Meech]

When you move to a plane, you can't control which 'side' you are looking at. This is particularly problematic for the block plane when we are assigning vertices as 'top left', 'top right' etc.... We may be looking down on the block surface from outside, or up at the block face from inside. This means that assignments can look flipped vs the 3D view, which can be challenging to understand first time.

• Ideally we would always look down on the block plane from outside
• Otherwise (probably the easiest option) is to provide a 'flip view' option in the right click menu, that flips your current view to look at the plane from the other side

[K-Meech]

Related to #7