[ui] "About" dialog: Fix some display issues

[cbentejac]

Description

This PR removes some code from the "About" dialog that was not working anymore and fixes the following:

• Display (correctly) the icon to access AliceVision's website;
• Replace "GitHub"'s icon, which was a 32x32px png image, with an SVG one to prevent resolution issues (like the one reported in HiDPI: github logo in about #2636);
• Use the same color for all the icons in the dialog.

Prior to this PR, the "About" dialog looked like this:

With it, it now looks as follows:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.93%. Comparing base (f8f549a) to head (728f2ea).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2640   +/-   ##
========================================
  Coverage    69.93%   69.93%           
========================================
  Files          121      121           
  Lines         7088     7088           
========================================
  Hits          4957     4957           
  Misses        2131     2131           

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