Add vendor agnostic default window size and position settings to the App Directory app definition format

## Enhancement Request
Most container/Desktop Agent vendors support the configuration of a default window size (in pixels) and position (in pixels or positional hints such as 'center') for applications, which can currently be specified in a host manifest. Given the common occurrence and form of these configurations, it may be advantageous to create vendor-agnostic properties for these settings within the application definition. Adherence to the values specified in any such properties should be optional but recommended (SHOULD).

### Use Case:
- Default window sizes and positions for applications are usually supported by Desktop Agents/containers, however, there is currently no vendor-agnostic way of specifying these settings in an AppD app definition.
- Providing vendor-agnostic support for common settings saves publishers of AppD records from having to provide multiple hostManifests defining behaviour in proprietary formats.

### Additional Information
Note: the Web Application Manifest specification does NOT support size and position properties, instead it defines display modes (e.g. "fullscreen", "standalone", "minimal-ui", "browser") and orientation ( "any", "natural", "landscape", "portrait", "portrait-primary", "portrait-secondary", "landscape-primary", or "landscape-secondary"). These are better suited to mobile and tablet applications than those used on a financial services desktop. Further, the form factor of a desktop container is often different to that of a desktop browser (with more applications sharing the screen at the same time), leading to different requirements for the initial display of a newly launched application.