Fallback on background callback function names if source cannot be found

[corebit-nl]

Additional patch to #2922 to fully fix #1885 . Re-submitted version of #3072 .

This MR adds a patch made in #2922 to similar code, to make background callbacks fully work in compiled binaries (eg. PyInstaller).

Contributor Checklist

• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[T4rk1n]

💃

[corebit-nl]

Hi @T4rk1n @ndrezn @emilykl ! Are there any remaining actions I should perform to get this PR merged? Thanks!

[camdecoster]

@corebit-nl could you please add a changelog entry? With that, we can get this merged.

[corebit-nl]

@corebit-nl could you please add a changelog entry? With that, we can get this merged.

I did not find the time until now to do so, but I see you already did add a message. Thanks for your effort on getting this merged!