Pose Sliding tools improvements

### Executive Summary

This patch improves pose sliding tools: Breakdowner, Push, and Relax.

The problems with the old tools:
- **No UI**. Even though it's clear how the tool works when you move the mouse, there is no visual indication as to the actual scale of the movement required.
- **No mouse wrapping**. This means that you have to carefully plan the initial position of the mouse when using the tools, because that impacts which values are available. This makes it hard to use it from the menu, as that forces the mouse to be in a certain position.
- **No precision mode** by pressing shift.
- **No overshoot protection**, so it's very easy to go below 0% or above 100%.

These problems are **all solved in this patch**.

##### Demo video

Here {key Shift E} is used to start the Pose Breakdowner.

{F9929705}

### Original description

**Problem**
The existing Push, Relax and Breakdowner tools are great, but the fact that they use the whole area range is less than ideal.
In theory, the tools support going beyond 0% and 100% to create an overshoot, but since they use the whole are, you might be blocked by the edges of the screen.

**Solution**
Set a pixel value as defined distance from 0% to 100% (scales with gui)
To indicate that, draw a UI under the cursor.
The percentage is now calculated accumulative to enable precision support when holding SHIFT

Info about new modifier keys is printed to the workspace status
{F9891261}

Update 2020_10_12: All lines have outlines now and handle is orange for better readability on white.
Update 2021_01_18: Static indicators moved to bottom. tick spacing now every 10%
Update 2021_03_14: Remove the slider and replace with a simple percentage indicator. (Cursor not visible in screenshot)
Update 2021_04_11: Brought the slider back. Spawns centered at the bottom of the screen
{F9929709}

By default the percentage is clamped to 0-1. But by pressing E you can enable overshoot.
Update 2020_10_12: slider range in overshoot mode is now 150% to better indicate that it can go further
Update 2021_03_14: Since the slider was removed, the percentage amount is simply displayed under the cursor
Update 2021_04_11: Overshoot now scrolls in place
{F9929712}

**Limitations and future improvements**
As mentioned in the comments below it would be a good idea to write a more general version of the slider to be used in various areas of Blender.

**Build**
[[ https://blender.community/c/graphicall/dqbbbc/ | Windows build on graphicall ]]

**Edit**
Split off the operator into it's own patch D9137

Reviewed By: zeddb, brecht, Severin, looch

Maniphest Tasks: T81836

Differential Revision: https://developer.blender.org/D9054