Fixes & tests to improve coverage & resolve bugs related to untested code

[MyreMylar]

Testing coverage was dropping a bit over recent release so I thought I'd exert some effort to improve it as it usually turns up a few bugs.

[codecov]

Codecov Report

Merging #280 (d864b8f) into main (12d7122) will increase coverage by 1.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #280      +/-   ##
==========================================
+ Coverage   91.38%   92.42%   +1.03%     
==========================================
  Files          70       70              
  Lines        9698     9691       -7     
==========================================
+ Hits         8863     8957      +94     
+ Misses        835      734     -101     

Impacted Files	Coverage Δ
pygame_gui/elements/ui_vertical_scroll_bar.py	100.00% <100.00%> (+5.41%)	⬆️
pygame_gui/windows/ui_colour_picker_dialog.py	95.02% <100.00%> (+2.01%)	⬆️
pygame_gui/windows/ui_file_dialog.py	87.38% <100.00%> (+4.28%)	⬆️
pygame_gui/core/ui_element.py	95.83% <0.00%> (+0.46%)	⬆️
pygame_gui/core/ui_font_dictionary.py	92.64% <0.00%> (+1.47%)	⬆️
pygame_gui/elements/ui_image.py	96.42% <0.00%> (+7.14%)	⬆️
pygame_gui/ui_manager.py	92.18% <0.00%> (+7.81%)	⬆️
... and 2 more

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