Add favicon url download dialog

[peshevv]

Previously, it was a bit cumbersome to get a favicon from a URL that you did not already have from an entry. You would need to temporarily create an entry and download the favicon with the URL before discarding it.

To make this easier, I took the "download favicon" button on the Icon selection page that previously would appear if the URL field was filled into the entry, and converted it so that it is always visible and provides a dialog popup that allows the user to download favicons. This also makes it so that you can download icons that may not already be in your icon list when editing a group.

Screenshots

Testing strategy

Tested manually on Linux. I reused the existing favicon download implementation, so the code change just involved creating a new Dialog and altering some connection endpoints. I confirmed that the Dialog handles valid and invalid URLs properly.

Type of change

• ✅ New feature (change that adds functionality)

[droidmonkey]

The dialog should be pre populated with the entry's url

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.

Project coverage is 64.22%. Comparing base (12d16f6) to head (8c2fe39).
Report is 546 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/gui/EditWidgetIcons.cpp	42.86%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6614      +/-   ##
===========================================
- Coverage    64.22%   64.22%   -0.00%     
===========================================
  Files          336      336              
  Lines        42369    42377       +8     
===========================================
+ Hits         27209    27214       +5     
- Misses       15160    15163       +3     

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[droidmonkey]

I completely rewrote the code here and moved the text box directly into the Edit Icon widget. I also took this opportunity to cleanup the buttons.