Simplifying `lapce_ui::settings::ThemeSection::new`

[JustForFun88]

This pull request simplifies the lapce_ui::settings::ThemeSection::new function by removing the allocation of two intermediate vectors (one for Vec<&str>, the second inside Itertools::sorted method).

Also, since the colors are just Strings and it's not important to preserve their order, the Vec::sort_unstable method is now used instead of the Vec::sort method (inside Itertools::sorted method), since this method is usually faster and does not require any additional allocations.

[codecov-commenter]

Codecov Report

Merging #1917 (75c77a9) into master (97d626d) will decrease coverage by 0.00%. The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master   #1917      +/-   ##
=========================================
- Coverage    8.49%   8.49%   -0.01%     
=========================================
  Files         130     130              
  Lines       56586   56578       -8     
=========================================
- Hits         4805    4804       -1     
+ Misses      51781   51774       -7     

Impacted Files	Coverage Δ
lapce-ui/src/settings.rs	0.00% <0.00%> (ø)
lapce-data/src/keypress/keypress.rs	73.02% <0.00%> (-0.21%)	:arrow_down:

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