[Qt6 migration] 4.0.0 doesn't see user plugins

[DmitryArefiev]

Steps to reproduce

1. Add
   abc_import-4.0.0.zip
   batch_export-master.zip to c:\Users_user name_\Documents\MuseScore4Development\Plugins\
2. Run MuseScore

Screenshots/Screen recordings

bandicam.2024-02-23.18-02-56-875.mp4

MuseScore Version

4.4.0

Regression

Yes, this used to work in MuseScore 4.3

Operating system

Windows11/Mac13.6

Additional context

No response

[cbjeukendrup]

The reason is that these plugins are not valid for Qt 6; they need to be rewritten. Perhaps, instead of ignoring them completely, we could display these plugins as "invalid", so that the user understands what's going on. Maybe we can even catch the error messages that Qt gives when trying to load these plugins, and somehow convey these to the user, so that the user can try to fix them.

[bkunda]

Okay, so a simple design solution here would be to still show the "invalid" plugins with a reduced opacity, and perhaps a warning sign label.

But at this stage the 0-development solution (preferable from a time POV) though would be just to ensure we have this covered in our documentation. @shoogle could you please add this as an FAQ on the website? (E.g. "Why aren't some of my plugins showing up in MuseScore Studio").

[shoogle]

Note for plugin authors: these commits show the changes required to get plugins working with Qt 6.

[Jojo-Schmitz]

The good part is that these 2 plugins never really worked for Mu4, due to #13593/#20822
And won't in Mu4, onless that issue gets fix resp. that PR gets merged...

But these tips don't seem to help much, I can only get a blank window for batch_convert