feat: Improved country picker

[g123k]

Hi everyone,

Here is some improvements for the country picker:

• There is now a title (I don't use the one provided by the SmoothDialog, because the style doesn't match)
• There are now dividers between items
• Running a research will now highlight parts
• I also fix an issue, where the previous input was persisted when closing/reopening the dialog

A video is better than words: CountryPicker.webm

And a screenshot, just in case:

This part of #4230 to improve the walkthrough / onboarding.

[codecov-commenter]

Codecov Report

Merging #4300 (644fd96) into develop (296a0d2) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #4300      +/-   ##
===========================================
- Coverage    10.82%   10.79%   -0.04%     
===========================================
  Files          288      289       +1     
  Lines        14264    14308      +44     
===========================================
  Hits          1544     1544              
- Misses       12720    12764      +44     

Impacted Files	Coverage Δ
...ckages/smooth_app/lib/helpers/keyboard_helper.dart	0.00% <0.00%> (ø)
...oth_app/lib/pages/onboarding/country_selector.dart	0.60% <0.00%> (-0.22%)	⬇️

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[M123-dev]

Looks great thanks @g123k