Reset splitter sizes on database unlock

[droidmonkey]

• Attempt to avoid issue with splitters not being appropriately calculated because the main window isn't sized yet. This can happen if the main window is hidden when the database is loaded and the splitter sizes are not recorded in the config file.
• Fixes #11013

Improve related splitter UX as well:

• Prevent group pane from being hidden just by dragging. Introduce new View menu setting to hide the group pane.
• Replace the preview panel "close" icon with a "collapse down" icon making the intention clearer.
• Better organize the view menu

Testing strategy

Manual testing is pending...

Type of change

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 7 lines in your changes missing coverage. Please review.

Project coverage is 63.74%. Comparing base (95bae83) to head (f948a41). Report is 24 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/gui/DatabaseWidgetStateSync.cpp	91.18%	3 Missing :warning:
src/gui/DatabaseWidget.cpp	77.78%	2 Missing :warning:
src/gui/MainWindow.cpp	60.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #11014      +/-   ##
===========================================
+ Coverage    63.72%   63.74%   +0.02%     
===========================================
  Files          362      362              
  Lines        38002    38028      +26     
===========================================
+ Hits         24215    24240      +25     
- Misses       13787    13788       +1     

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[droidmonkey]

I want to write unit tests for this before I merge

[droidmonkey]

@phoerious can you take another look? I added a commit that disables collapsing the group view and instead adds a view menu action to explicitly show/hide the group view. The behavior is similar to the preview panel.