Fix event emission when setting property on a state device

[tlambert03]

fixes #104

also creates and moves some tests. The only new thing in the PR is the 6 lines in _mmcore_plus

[codecov]

Codecov Report

Merging #105 (d71aa79) into main (2fcf5fc) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   89.46%   89.48%   +0.02%     
==========================================
  Files          23       23              
  Lines        1035     1037       +2     
==========================================
+ Hits          926      928       +2     
  Misses        109      109              

Impacted Files	Coverage Δ
pymmcore_plus/core/_mmcore_plus.py	89.17% <100.00%> (+0.06%)	⬆️

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