Deprecation of QSharedMemory and QSystemSemaphore

[Kai Uwe Broulik]

Hi everyone,

at Qt Contributor’s Summit it came up that 
QSharedMemory/QSystemSemaphore are fundamentally broken and will be 
binned. [1]

I found that we have a few users of those classes according to lxc:

QSystemSemaphore:
* Solid optical disc handling
* Digikam server starter

QSharedMemory:
* Marble
* KMemFile used by KSycoca
* Solid again
* Snorenotify
* Amarok
* Kdenlive

At a glance it looks like some (e.g. Solid) can probably use QLockFile 
or are Linux-specific and can use memfd

Qt folks are aware of QtSingleApplication but we couldn’t really speak 
of the other use cases, in particular KSycoca.

Cheers
Kai Uwe

[1] 
https://lists.qt-project.org/pipermail/development/2023-November/044680.html