fix(SDK): support SteamVR Plugin version 1.2.0
fix(SDK): support SteamVR Plugin version 1.2.0

The SteamVR Plugin has been updated to version 1.2.0 which removed the
method `SteamVR_Util.Event.Listen`. Since it has not been deprecated it
now causes errors to be thrown in VRTK as this method was used within
the SteamVR Controller SDK.

This fix updates the code to use the new API when found, otherwise it
will fall back to the previous API.

thestonefox/SteamVR_Unity_Toolkit,mattboy64/SteamVR_Unity_Toolkit,gpvigano/SteamVR_Unity_Toolkit,gpvigano/VRTK,thestonefox/VRTK,Innoactive/IA-unity-VR-toolkit-VRTK,Fulby/VRTK,gomez-addams/SteamVR_Unity_Toolkit,adamjweaver/VRTK,red-horizon/VRTK,virror/VRTK,TMaloteaux/SteamVR_Unity_Toolkit