Fix T83161: Crash when moving ruler endpoints and opening or closing the toolshelf

`invert_snap` could be called before `snap_gizmo->keymap` was found.

Use the lazy initialization in `invert_snap` then.

Fix T83177: Industry Compatible keymap: MMB-dragging to transform engages axis-constraining on release

Release confirm did not consider modal keymap events.

Fix transform snap mixed with incremental

The incremental was taking priority but the other elements are mandatory.

Fix T83092: Direction of rotation with View orientation changed in 2.91

The change was intentional so that the orientation matrices match
(`rv3d->viewinv` becomes equal to the orientation matrix).

But, although in a projection matrix the Z axis is negative, this
should not be so from the user's point of view.

So the solution here is to negate the Z axis when the View orientation
is chosen.

This affects all modes, but is only evident for rotation.

---
Another change here is to use the final rotation value (`values_final`)
for the gizmo drawing since this value can be changed by the mode.

Transform: Improve event detection for AutoConstrain

The event "value" is not really required since this operation works as a toogle.

This change cleans and simplifies the code.

Fix T83020: Transform: AutoConstraint being confirmed without releasing the MMB

The transform modifiers are confirmed by releasing any button.

Thus, the operation can be falsely confirmed if the button that launched
the operation is released after the modifier has been activated.

Previously the events that confirmed the modifiers were hardcoded.

An option to fix this would be to add custom confirmation keyitens for
specific modifiers. But this can be a bit confusing and would make the
modal keymap even bigger.

So the solution here is to skip the button that launched the operation
when confirming the modifier.

RNA Armature: Improve the description of AxisRollFromMatrix

The roll value may not be as expected when a matrix is not orthogonal
or has a negative determinant.

This can lead to confusion as seen in T82930.

Therefore, make it clear that this is a limitation and that a value for
the roll is somewhat indeterminable in these cases.

This fixes T82930

Limit this feature to Move, Rotate and Resize modes

Transform: New feature to set a custom 'Snap With'

Ref T66484

Basically the idea is to use a modal keymap to change the current Base Point
defined by `Snap With` by one that can be chosen through snapping.

{F9170047}

I prefer to avoid using any of the modifier buttons (Shift, Alt, Ctrl) as
they can be used in the future to allow navigation during transform operations.

So the shortcut chosen in this patch is the {key B}.

Note:
This feature is not enabled if the scene snap option is Incremental or grid.

Maniphest Tasks: T66484

Differential Revision: https://developer.blender.org/D9415

# Conflicts:
# source/tools

Enable modes according to the number of times the button is pressed