Replace date/time pickers with text input

[VMelnalksnis]

Fixes #301.

[codecov]

Codecov Report

Merging #378 (a84f7ab) into master (ad7a0a9) will decrease coverage by 0.26%.
The diff coverage is 3.57%.

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
- Coverage   67.66%   67.40%   -0.27%     
==========================================
  Files         248      249       +1     
  Lines        6826     6853      +27     
  Branches      490      501      +11     
==========================================
  Hits         4619     4619              
- Misses       2041     2068      +27     
  Partials      166      166              

Impacted Files	Coverage Δ
...hade.Interfaces.Avalonia.Core/DateTimeConverter.cs	0.00% <0.00%> (ø)
...erfaces.Avalonia.Core/DesignTime/DesignTimeData.cs	97.59% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ad7a0a9...a84f7ab. Read the comment docs.