Nexxt

**Fixes**:
- fix variable editors always filled with empty values instead of the previous values

**Changes**

- Implement variable substitutions on various levels; composite filter instances may override variables as needed
- fix https://github.com/ifm/nexxT/issues/53 (the C++ signature of signal propertyChanged() has changed)
- https://github.com/ifm/nexxT/issues/59 (--single-threaded, --disable-unload-heuristic, --no-profiling)

**Fixes**

- Fix overflow errors when loading recordings containing zero timestamps

Decrease deadlock probability
Switch to PySide 6.5.3 and add workarounds to new bugs introduced in this version
Fix restoring positions of MDI Subwindows in the main window
Make invokeMethod() more efficient by calling the requested slot directly without making a connection