Add buttons to save image of results visualization

[lkeegan]

• Add Trial Image and Condition Image buttons to results widget
  • Screenshots visualization of results
  • Prompts user for a filename to save the screenshot as a png image
• Add return_screenshot bool arg to display_drawables and display_results
  • If true, do a single flip and then return image of window
  • Default: false
• resolves #226

[codecov[bot]]

Codecov Report

Merging #230 (d193df7) into main (07fd116) will decrease coverage by 0.18%. The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
- Coverage   89.50%   89.32%   -0.18%     
==========================================
  Files          19       19              
  Lines        1467     1499      +32     
==========================================
+ Hits         1313     1339      +26     
- Misses        154      160       +6     

Impacted Files	Coverage Δ
src/vstt/results_widget.py	93.40% <82.35%> (-6.60%)	:arrow_down:
src/vstt/vis.py	94.31% <93.33%> (+0.20%)	:arrow_up:

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication