add application_name variable

[tlambert03]

adds a variable magicgui.application.APPLICATION_NAME that all backends must use to name their app.

[codecov]

Codecov Report

Merging #55 (8c8e6e3) into master (5ee09ae) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   77.48%   77.57%   +0.09%     
==========================================
  Files          16       16              
  Lines        1790     1793       +3     
==========================================
+ Hits         1387     1391       +4     
+ Misses        403      402       -1     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/application.py	86.11% <100.00%> (+1.26%)	⬆️
magicgui/widgets/_protocols.py	74.49% <100.00%> (+0.67%)	⬆️

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