Lpe bool hide operand

Split bool LPE into two separate LPEs:

  • lpe-bool: classical bool ops for two closed paths
  • lpe-cut-path: cut a path or path set (usually open) on a closed path

The split makes both the UI and the code simpler, and easier to understand. The two LPEs are quite orthogonal.

Also automatically hide the operand path (properly undone when LPE is removed or disabled).

Only thing missing is a check if the path and operand path are closed/open as they should be and report an error if not. I couldn't find a way to check if a pathvector is closed.