When a new payload file would be saved, the subsequent flight would always throw an exception (until Retribution would be restarted) because the payload_cache wasn't updated until now.
Added a check for the existence of a payloads folder. Also made the _save_payloads lookup and path generation the same as _backup_payloads method to have consistency.
This appears to be incompatible with pyinstaller. I get the following
when trying to run the executable generated with pyside6:
```
Traceback (most recent call last):
File "qt_ui\main.py", line 29, in <module>
File "PyInstaller\loader\pyimod03_importers.py", line 476, in exec_module
File "qt_ui\windows\QLiberationWindow.py", line 28, in <module>
File "PyInstaller\loader\pyimod03_importers.py", line 476, in exec_module
File "qt_ui\widgets\map\QLiberationMap.py", line 11, in <module>
ImportError: could not import module 'PySide6.QtPrintSupport'
```
Loadout selection no longer has two (disagreeing) implementations. What
the UI shows is now what the miz will have.
We now store the chosen layout in the Flight *always*, not just for
custom loadouts. This means that we do loadout lookups at the start of
each turn, but the data is cached in pydcs.
Era-specific loadout degradation is still done at generation (and
presentation) time. This is so that players can toggle that option and
have it affect the *current* turn, rather than the next one.
A user can now modify the existing default loadout, rather than starting from scratch.
Also adds WIP handling for removed pylons, which looks like it may need some PyDCS work.
Also fixes the F-14B default loadouts for everything OTHER than fighter sweep again.
The default loadout is now displayed whenever the custom loadout checkbox is unchecked.
The custom loadout is reset when the custom loadout is checked, to force the user to be prescriptive about the loadout.
Contributes-to: Khopa/dcs_liberation#725
Follow up work:
* Data entry. I plan to do the air-to-air missiles in the near term. I
covered some variants of the AIM-120, AIM-7, and AIM-9 here, but there
are variants of those weapons for each mounting rack that need to be
done still, as well as all the non-US weapons.
* Arbitrary start dates.
https://github.com/Khopa/dcs_liberation/issues/490
