Use Share button on DS4 controllers (ViGEm DS4_BUTTON_SHARE) as BACK action

[aseligmann]

Description

It is currently not possible to register a "BACK" action (Select / Share button press) when Sunshine is configured to emulate a DS4 controller on Windows.
This seems to be due to the BACK action being triggered by the touchpad button press instead.
When testing I was not able to register a BACK action using the touchpad, since as soon as the touchpad is pressed it is treated as a mouse input instead.

This PR is a simple fix for the "BACK" functionality on DS4 controller emulation when using a DS4 or DualSense controller, which maps it to the Share button, and unmaps the non-functional touchpad press.

(It is up to interpretation whether this is a breaking change, as it unmaps the touchpad press despite the touchpad press seemingly being incorrectly registered.)

Screenshot

N/A

Issues Fixed or Closed

• Fixes DualSense controller SHARE key not working #799
• Most likely related to DS4 - Select button treated as Start #965
• Possibly related to Dualsense touchpad is read as 5 different inputs at the same time #1064

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

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