fix(NcAppContent): adapt to new emitted event object with splitpanes ^4.0.0

[wofferl]

☑️ Resolves

• Fix [NcAppContent] pane resize broken with 9.0.0-rc.1 #6931

From Splitpanes Release Notes

Version 4.0.0

Refactored all the emitted events to always return a single object containing as much information as possible. E.g. event, index, pane, prevPane, nextPane, panes.

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 43.25%. Comparing base (9d3b7ac) to head (9321a93).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/NcAppContent/NcAppContent.vue	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6950      +/-   ##
==========================================
+ Coverage   43.22%   43.25%   +0.03%     
==========================================
  Files         254      254              
  Lines       18942    18930      -12     
  Branches      948      948              
==========================================
+ Hits         8187     8189       +2     
+ Misses      10641    10629      -12     
+ Partials      114      112       -2     

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