Add table.changed event emitter

[tlambert03]

closes #205 by adding a table_widget.changed eventEmitter.

with table_widget.changed.connect(callback), the callback function will receive an event (whenver any item of the table changes in the GUI), and the event.value will be a dict:
{'data': x, 'row': int, 'column': int, 'column_header': str, 'row_header': str}

[codecov]

Codecov Report

Merging #209 (699fb19) into master (e021216) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   90.04%   90.11%   +0.06%     
==========================================
  Files          29       29              
  Lines        2994     3005      +11     
==========================================
+ Hits         2696     2708      +12     
+ Misses        298      297       -1     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.84% <100.00%> (+0.36%)	⬆️
magicgui/widgets/_protocols.py	99.44% <100.00%> (ø)
magicgui/widgets/_table.py	96.85% <100.00%> (+0.03%)	⬆️

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