refactor(AdaptiveQuality): move keyboard shortcuts into its own class
refactor(AdaptiveQuality): move keyboard shortcuts into its own class

This is more consistent with how the other constants are used. It's now
also easier to change them if needed.

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