add window_specification module and move WindowSpecification definition to that

[junkmd]

The WindowSpecification is referred from both Application and Desktop, but it was defined in the base_application module.
Thus, the Desktop needs to import the WindowSpecification from base_application module. It makes an undesirable dependency.

Additionally, WindowSpecification is a concrete class, and defining it in a module named base_... feels somewhat out of place.

I moved the definition of WindowSpecification to a module named window_specification, similar to how BackEnd in the backend module.

[codecov]

Codecov Report

Merging #1326 (4a52153) into atspi (bf7f789) will decrease coverage by 6.30%.
The diff coverage is 93.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##            atspi    #1326      +/-   ##
==========================================
- Coverage   94.15%   87.86%   -6.30%     
==========================================
  Files          60       61       +1     
  Lines       23036    23040       +4     
==========================================
- Hits        21689    20243    -1446     
- Misses       1347     2797    +1450