Fix styling of database tag view in main window

[droidmonkey]

• Fix a bunch of styling issues with the tags interface
• Also allow collapsing the entire left splitter instead of a dedicated option to hide the group view.

Screenshots

BEFORE:

AFTER:

Testing strategy

Tested using light and dark themes

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)

[codecov-commenter]

Codecov Report

Merging #7436 (6e094ab) into develop (d10c5a4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #7436   +/-   ##
========================================
  Coverage    64.33%   64.33%           
========================================
  Files          339      339           
  Lines        43216    43210    -6     
========================================
- Hits         27802    27799    -3     
+ Misses       15414    15411    -3     

Impacted Files	Coverage Δ
src/core/Config.cpp	89.70% <ø> (ø)
src/core/Config.h	100.00% <ø> (ø)
src/gui/MainWindow.cpp	72.53% <ø> (+0.05%)	⬆️
src/gui/group/GroupView.cpp	73.63% <ø> (-0.60%)	⬇️
src/gui/DatabaseWidget.cpp	61.87% <100.00%> (+0.13%)	⬆️

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