fix: Table.data support for negative indices

[Antyos]

Qt native Table _mgui_get_cell() and _mgui_set_cell() now support negative indices and will raise an IndexError if invalid indices are specified.

Previously, getting or setting to an invalid index would do nothing. Raising an IndexError is probably more useful, but may break some existing code.

Fixes #735.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.32%. Comparing base (2d985e7) to head (e070273).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
+ Coverage   89.27%   89.32%   +0.05%     
==========================================
  Files          40       40              
  Lines        4821     4835      +14     
==========================================
+ Hits         4304     4319      +15     
+ Misses        517      516       -1     

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