Removed explicit exit() from Textual-related classes and functions

[tieneupin]

When referring to the Textual demo app, it does not once call an explicit exit(), relying instead on self.exit() to close itself. This seems to be the cause of the raised I/OErrors when we try to quit the Murfey app.

Credits to: @stephen-riggs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.38%. Comparing base (9e7539a) to head (4c6eeb9).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
+ Coverage   28.37%   28.38%   +0.01%     
==========================================
  Files          71       71              
  Lines        9608     9603       -5     
  Branches     1284     1284              
==========================================
  Hits         2726     2726              
+ Misses       6788     6783       -5     
  Partials       94       94              

[tieneupin]

This PR should be merged together with, but before PR #360 in order to properly fix MSYS2 functionality.