OpenVR SDK 1.0.3:

General:
* Added preliminary Vulkan support

* Added Button ID k_EButton_ProximitySensor. Use this for raw access to a proximity sensor if there is one.

* Fixed spelling error in EVROverlayError, "VROVerlayError_KeyboardAlreadyInUse"

IVRSettings:
* Improved default value handling.  Defaults are no longer passed with each individual call to GetBool, GetInt32, GetFloat, and GetString. Instead, system-wide defaults are specified in "default.vrsettings" (in the resources/setting/ directory of either the runtime or the driver_xxx directory.) If not explicitly defined, the default will be false, 0, 0.0 or "" with an error of VRSettingsError_UnsetSettingHasNoDefault.

* Added explicit EVRSettingsError for VRSettingsError_JsonParseFailed

[git-p4: depot-paths = "//vr/steamvr/sdk_release/": change = 3647965]