Knife: Preserve right click cancel functionality

Currently, the knife does not use right click cancel.
It causes users to accidentally delete entire cuts easily.

This patch allows right click cancel when no cuts have been made.
This makes it consistent with other tools when switching between them.
More info: https://devtalk.blender.org/t/gsoc-2021-knife-tool-improvements-feedback/19047/175?u=hobbesos

Cleanup: Confusion with knife xray functionality

Fix: Knife unused function warning

Fix: Knife measurements broken when a cut point is in space

Knife angle measurements were mis-aligned if a cut point was in space.
Specifically, the arc drawing would not match with the cut line.

Fixed by removing a correction for kcd->prev.cage.
This correction was originally added for panning with measurements to work.
In hindsight it is not needed and only introduces issues like this.

Fix: Knife undo with no cut segments left

Now if a user presses the knife tool undo key when there are no more cut segments to undo, the operator exits. Previously, it did nothing.

Cleanup: Removed redundant if macro

Knife: Expose XYZ axis locking in modal keymap

A small quality of life improvement that will allow users to change the keys used for axis locking.

Applying patch D12600, GSOC Knife Tools branch

This adds constrained angle mode improvements,
snapping to global and local orientation,
visible distance and angle measurements,
undo capability,
x-ray mode,
multi-object edit mode.

See https://developer.blender.org/D12600 for more details.

Note: this project moved some of the default keymappings
around a bit, as discussed with users in the thread
https://devtalk.blender.org/t/gsoc-2021-knife-tool-improvements-feedback/19047
We'll change the manual documentation in the next couple of days.

Merge branch 'master' into soc-2021-knife-tools

Fix: Knife keymap failed unit test