Download favicon in the background after credential add

[varjolintu]

Downloads a favicon in the background if set by keepassxreboot/keepassxc-browser#1472.
The feature is only available via the browser extension.

Testing strategy

Manually.

Type of change

• ✅ New feature (change that adds functionality)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 64.25%. Comparing base (d16fc2d) to head (da9eaad).
Report is 547 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/gui/IconDownloaderDialog.cpp	0.00%	15 Missing ⚠️
src/gui/DatabaseWidget.cpp	0.00%	8 Missing ⚠️
src/browser/BrowserAction.cpp	0.00%	3 Missing ⚠️
src/browser/BrowserService.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7179      +/-   ##
===========================================
- Coverage    64.26%   64.25%   -0.01%     
===========================================
  Files          336      336              
  Lines        42344    42369      +25     
===========================================
+ Hits         27212    27223      +11     
- Misses       15132    15146      +14     

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