Implement "Overwrite attachment" confirmation dialog.

[libklein]

Fixes #7054.

Adds a confirmation dialog if adding a file attachment to an entry would cause an existing attachment to be overwritten.

Screenshots

Testing strategy

Tested with a test database.

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)

[libklein]

Would you guys mind adding the "Hacktoberfest" label?

[codecov-commenter]

Codecov Report

Merging #7083 (6b496d0) into develop (9aa30c4) will increase coverage by 0.18%.
The diff coverage is 27.33%.

❗ Current head 6b496d0 differs from pull request most recent head fe7edf9. Consider uploading reports for the commit fe7edf9 to get more accurate results

@@             Coverage Diff             @@
##           develop    #7083      +/-   ##
===========================================
+ Coverage    63.77%   63.95%   +0.18%     
===========================================
  Files          330      330              
  Lines        41857    41773      -84     
===========================================
+ Hits         26691    26712      +21     
+ Misses       15166    15061     -105     

Impacted Files	Coverage Δ
src/browser/BrowserAction.h	100.00% <ø> (ø)
src/browser/BrowserService.cpp	27.42% <0.00%> (-0.60%)	⬇️
src/browser/BrowserService.h	100.00% <ø> (ø)
src/browser/BrowserSettings.cpp	53.33% <ø> (+29.30%)	⬆️
src/browser/BrowserSettings.h	100.00% <ø> (ø)
src/format/HtmlExporter.cpp	0.00% <0.00%> (ø)
src/gui/PasswordGeneratorWidget.cpp	76.24% <0.00%> (-0.76%)	⬇️
src/gui/entry/EntryAttachmentsWidget.cpp	27.13% <0.00%> (-2.12%)	⬇️
src/browser/BrowserAction.cpp	22.44% <35.71%> (+0.97%)	⬆️
src/fdosecrets/objects/Item.cpp	85.81% <87.80%> (+0.41%)	⬆️
... and 4 more

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[droidmonkey]

I merged the two functions into one, much better! Also fixed up the wording to be more direct.