Path-tool improvements

[0SlowPoke0]

Part of #1870

Added features:

When double-clicking an anchor:

• Currently this swaps between smooth and sharp, but doesn't work if multiple handles are multi-selected to convert them all to smooth or sharp. If they're of mixed type, the specific one that's double clicked on should be the decider for the rest of them as to which they should all be swapped to.

• If a single handle is double clicked, and it has only one handle (the other is zero-length or doesn't exist), currently this converts it to sharp by removing the handle. But we want that to make the new handle colinear and extend it to an equal length as the present handle. Only if both handles are present, should double-clicking remove both handles like it does currently in that situation. And if both handles are gone, it should also do what it does now by restoring them as colinear.