XR Controller Support Step 1: Internal Abstractions for OpenXR Actions

Adds internal API for creating and managing OpenXR actions at the
GHOST and WM layers. Does not bring about any changes for users since
XR action functionality is not yet exposed in the Python API (will be
added in a subsequent patch).

OpenXR actions are a means to communicate with XR input devices and
can be used to retrieve button/pose states or apply haptic feedback.
Actions are bound to device inputs via a semantic path binding
(https://www.khronos.org/registry/OpenXR/specs/1.0/html/xrspec.html#semantic-path-interaction-profiles),
which serves as an XR version of keymaps.

Main features:

- Abstraction of OpenXR action management functions to GHOST-XR,
  WM-XR APIs.
- New "xr_session_start_pre" callback for creating actions at
  appropriate point in the XR session.
- Creation of name-identifiable action sets/actions.
- Binding of actions to controller inputs.
- Acquisition of controller button states.
- Acquisition of controller poses.
- Application of controller haptic feedback.
- Carefully designed error handling and useful error reporting
  (e.g. action set/action name included in error message).

Reviewed By: Julian Eisel

Differential Revision: http://developer.blender.org/D10942

source/blender/python/intern/bpy_app_handlers.c

@@ -74,6 +74,7 @@ static PyStructSequence_Field app_cb_info_fields[] = {
     {"version_update", "on ending the versioning code"},
     {"load_factory_preferences_post", "on loading factory preferences (after)"},
     {"load_factory_startup_post", "on loading factory startup (after)"},
+    {"xr_session_start_pre", "on starting an xr session (before)"},
 
 /* sets the permanent tag */
 #define APP_CB_OTHER_FIELDS 1