App: Rework handling of invalid file for better watch support

[mwestphal]

Describe your changes

• Rework handling of invalid files and keep track of the scene status
• Stop writing [INVALID] in the filename info, the log is available now instead
• Write [EMPTY] when the scene is empty
• Support watching an invalid file (fix for supported files stop being --watched if they fail to load #2246)
• Support watching a non-existent, unsupported or over max-memory file
• Better logic to avoin unwatching/rewatching all the time (fix for -watch flag can crash f3d.exe #2354)
• Add/Improve tests

Issue ticket number and link if any

Fix #2246
Fix #2354

Checklist for finalizing the PR

• I have performed a self-review of my code
• I have added tests for new features and bugfixes
• I have added documentation for new features
• If it is a modifying the libf3d API, I have updated bindings
• If it is a modifying the .github/workflows/versions.json, I have updated timestamp

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[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.18%. Comparing base (2c9afe5) to head (f1b18c5).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
application/F3DStarter.cxx	95.65%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2392   +/-   ##
=======================================
  Coverage   96.18%   96.18%           
=======================================
  Files         134      134           
  Lines       12114    12118    +4     
=======================================
+ Hits        11652    11656    +4     
  Misses        462      462           

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