Dialog: Add filter size argument, remove NULL termination

[Semphriss]

Description

This replaces the handling of the length of the filters arguments from null-terminated to a separate size argument, which should be less bug-prone.

I've also removed the case where if a user selected a file without selecting any filter, it would return the size of the array. It now returns -1, the same as when the platform doesn't support fetching the chosen filter. I'm not sure there's any difference that's relevant for the downstream developers.

Note: I've had to manually update the dynapi files to accomodate for the new arguments. Running gendynapi.py did not update the arguments, and it added two thread-related functions which seem to have been forgotten.

Existing Issue(s)

Closes #9693

[Semphriss]

While fixing Haiku compilation problems, I've also added some more checks to filter-related functions to make sure that attempting to convert NULL filters will result in an error, not a crash.

[slouken]

Merged, thanks!