[Reg 6.2->6.3] QML FileDialog lost ability to set initial filename

Using FileDialog with SaveFile:

with Qt 6.2, it was possible to set `currentFile` property to a default filename before calling .open() and it would set that filename as initial in the opened dialog.

In Qt 6.3, currentFile was deprecated and the alternative `selectedFile` is read-only.

Not only it is read-only, but the function call to actually set the filename was removed here: https://github.com/qt/qtdeclarative/commit/13399bd54d084ed837ec061ca9315dbd173f3b48#diff-d64ac4854b6856e210361095c9d3abce9f15a12bcaf182b0f26efcc9a130773cL251-L255

Please bring this functionality back.