Obtain an AccessibleProxy from any ObjectRef with ObjectRefExt

[luukvanderduim]

It is assumed that all objects in any server's tree implements the Accessible interface.
Therefore it would be very convenient to quickly obtain a AccessibleProxy on an object.

Bringing ObjectRefExt into scope allows users to call as_accessible_proxy on any ObjectRef to obtain an AccessibleProxy from the ObjectRef.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 83.74%. Comparing base (655be70) to head (85d7518).

Files	Patch %	Lines
atspi-proxies/src/accessible.rs	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #182      +/-   ##
==========================================
- Coverage   84.14%   83.74%   -0.41%     
==========================================
  Files          39       39              
  Lines        3097     3112      +15     
==========================================
  Hits         2606     2606              
- Misses        491      506      +15     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[luukvanderduim]

We cannot do the same with TryFrom<ObjectRef> for AccessibleProxy because the proxy builder needs a zbus::Connection as argument.

This is probably the best we can do.

[TTWNO]

This is a great usability improvement for users of atspi, and only adds a few lines of code.

Since this is somewhat non-trivial to test, I'm going to ignore codecov.