from user freedback it seems there are 2 use cases, both valid.
* Select geometry and cut the selection in half (as 2.4x worked)
* Point-to-point define the faces to cut, dont cut through everything (only cut what you see).
With the second, since you are already selecting the edges to cut and snapping to them. only cutting the selecting is limiting/annoying.
Modifying these options while the knife tool runs doesn't work well, so expose under 2 keys, K, Shift-K.
For now, just reusing some theme colors that are there for other reasons
but matched approximately the colors used now.
Except that cut lines are now purple-violet, so that they contrast
with grid and edges better, especially in wireframe.
seed the random number generator to at least make these operator repeatable.
I don't know why it is using random numbers at all, we should really not be
doing this in my opinion, but I don't understand why it's being done so won't
remove it.
problem was that BMesh had tessellation call when undo pushes were called.
if python called an operator with no undo push, tessfaces would not be created.
fix this by making it the responsibility of each editmesh operator to re-tessellate, as it is with notifiers and depsgraph.
added EDBM_update_generic() function to add notifier, tag for depsgraph update and optionally re-tessellate.
