Workspace integration improvements

[Al12rs]

• Fixed various erratic behaviour of tab header area when resizing
• Disabled Add Panel button when it is empty
• Closed Add Panel Drop Down when item selection is changed
• Fixed missing bottom border of Scroll left button in tab are
• Basic MainWindow layout and border adjustments to match new designs

This PR:

Latest design:

Stuff still needs moving around

[erri120]

Thanks, resizing feels much better now.

[Al12rs]

CI tests fail with failed to open database file again

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a0ebd00) 0.00% compared to head (d0d4250) 55.85%.

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Additional details and impacted files

@@            Coverage Diff            @@
##           main     #912       +/-   ##
=========================================
+ Coverage      0   55.85%   +55.85%     
=========================================
  Files         0      617      +617     
  Lines         0    19694    +19694     
  Branches      0     1512     +1512     
=========================================
+ Hits          0    11001    +11001     
- Misses        0     8354     +8354     
- Partials      0      339      +339     

Flag	Coverage Δ
Linux	55.27% <100.00%> (?)
Windows	55.05% <100.00%> (?)
clean_environment_tests	55.84% <100.00%> (?)

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Files	Coverage Δ
...s.App.UI/Pages/LoadoutGrid/LoadoutGridViewModel.cs	63.41% <100.00%> (ø)

... and 616 files with indirect coverage changes