Swap seam/sharp edges operator

After few times, I made UV`s, I see, that i need such function. It will allow to reorder seams/sharps faster, and boost unwrapping speed especially with live-unwrap . It will allow to move existing seams to better place in two operations (select + swap) instead 4 (select - clear seam - select new edges - mark seam)

Operator is simple: for all selected edges, if it is already seam, then seam will be cleared. If there is no seam, edge will be marked as seam (or sharp).