Cleanup directories handling

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?
  This PR makes all the directory handling more straightforward.
  All directories computation is now decided in config.go through the getXxxxDirectory() functions.

• What is the current behavior?
  Directory structure and computation were spread across the code.

• What is the new behavior?

• Does this PR introduce a breaking change?

• Other information:

[codecov-commenter]

Codecov Report

Base: 11.40% // Head: 11.28% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (4dfde2f) compared to base (c724474).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #765      +/-   ##
==========================================
- Coverage   11.40%   11.28%   -0.12%     
==========================================
  Files          51       51              
  Lines        3910     3951      +41     
==========================================
  Hits          446      446              
- Misses       3409     3450      +41     
  Partials       55       55              

Flag	Coverage Δ
unit	11.28% <0.00%> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
certificates.go	0.00% <0.00%> (ø)
config.go	0.00% <0.00%> (ø)
hub.go	0.00% <0.00%> (ø)
icon/iconlinux.go	0.00% <0.00%> (ø)
main.go	2.34% <0.00%> (+0.01%)	⬆️
systray/systray.go	0.00% <0.00%> (ø)
systray/systray_real.go	0.00% <0.00%> (ø)

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