Add welcome screen for analytics consent

[Lezurex]

Until now, Matomo analytics were enabled by default without an actual opt-in. It was possible to opt-out in the settings but that's not really an acceptable solution, as was also pointed out in #809. However, we also still want to have as many users as possible contributing their anonymous usage data for us to optimise the user experience.

To solve this, we're now adding a welcome screen which is shown on first launch. It properly prompts for consent to collect anonymous usage analytics. Both the accept and decline buttons are placed and styled equally so the user has free choice. The welcome screen can be extended e.g. with some info about the project in the future.

Closes #809

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.57%. Comparing base (9af124d) to head (d662180).

Files with missing lines	Patch %	Lines
src/renderer/WelcomeScreen.tsx	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #918      +/-   ##
==========================================
+ Coverage   56.18%   56.57%   +0.39%     
==========================================
  Files          43       44       +1     
  Lines        1367     1377      +10     
  Branches      373      373              
==========================================
+ Hits          768      779      +11     
+ Misses        493      492       -1     
  Partials      106      106              

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