Bug 508512

SUMMARY
Some file selection dialogs show the project folder as the default folder, and some default to ~/Videos.

STEPS TO REPRODUCE
1. Create or open a project whose project folder isn't ~/Videos. Ideally a project file that lives in its project folder in some little-used portion of your drive, for clarity.
2. File->Open or "Add Clip or Folder" and observe that the dialog defaults to the project folder
3. OpenTimelineIO Export, OpenTimelineIO Import, Export Guides, or Import Guides and observe that the dialog defaults to, in my case, ~/Videos

OBSERVED RESULT
Inconsistency in the default folder of the file picker. 

EXPECTED RESULT
File picker should default to project folder in at least all of the cases above; and probably every file dialog in Kdenlive. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.16.1.arch1-1
KDE Plasma Version: 6.4.4-1
KDE Frameworks Version: ?
Qt Version: 5.15.17, 6.9.1.1

ADDITIONAL INFORMATION
The OTIO dialogs use `pCore->currentDoc()->projectDataFolder()`, and the Guides dialogs use `pCore->projectManager()->current()->projectDataFolder()`. Unless I'm wrong about project data folder = project folder, the dialogs should already be pointing to the right folder, but aren't.