Allow user to modify export counter value

[kbengs]

self.save_file is used for "save all to single file"
self.export_file is used for all other "exportmodes"

@petaflot would you like to test this before merge?

[codecov-commenter]

Codecov Report

Base: 71.77% // Head: 71.80% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (8cee08e) compared to base (46a4364).
Patch coverage: 82.92% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #794      +/-   ##
==========================================
+ Coverage   71.77%   71.80%   +0.03%     
==========================================
  Files          10       10              
  Lines        3479     3494      +15     
==========================================
+ Hits         2497     2509      +12     
- Misses        982      985       +3     

Impacted Files	Coverage Δ
pdfarranger/pdfarranger.py	68.66% <82.92%> (+0.15%)	⬆️
pdfarranger/core.py	86.63% <0.00%> (-0.26%)	⬇️

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[petaflot]

probably didn't test it the way I was supposed to (patched manually), but from what I've seen it works fine.

suggestion : IMHO it would be nice that "ctrl+shift+e" should export+save+delete selection. any thoughts?

[kbengs]

Thanks for testing. Not sure about "export+save+delete" But if it's done well and documented maybe it could be merged. "ctrl+shift+e" is already used though.