Note that the code for bisect is faster & simpler then boolean, for a trim tool we would probably want to adjust the functionality so it only trims regions the line segment touches.

I refactored the whole tool, now it should be easier to add more trimming modes in the future (polylines, regular polygons, beziers, snapping...) as it can cut any arbitrary polygon. I included box trimand lasso trim using this code.
The functions that generate the mesh for the boolean operation can also be reused in other modes if needed as they don't have any sculptspecific code.

I also included line trim using bisect and edgenet. Right now it is the only mode stable and fast enough for master. Maybe we can include the line trim tool enabled in the toolbar and leave box trim and lasso trim as experimental until the new boolean code is done.

Rather than 3 separate tools, could this be combined into one tool with three tool options?

@William Reynish (billreynish) It may require some refactor as the box mode needs to use the modal keymap and the other two don't. If this can be fixed, then yes, it should be easy to refactor and group these tools. But the current box mask and lasso mask operators are also split into two tools in the same submenu.