Add an option to choose frame rate when importing image sequences

Importing image sequences could be improved by allowing the option to choose a frame rate to be interpreted at. When working in projects that require mixed rates (e.g. if adding cel animation to live action and the cel is animated on 2s) having image sequences import at the standard rate

Hernán Lindenbaum suggested having a frame rate box next to the checkbox for marking as an image sequence in the import dialog - this could show the project / system default value but be able to be overwritten at this point. This seems like the best place for it, as it would be accessible and visible in a logical place.

The big time saving would be in not having to manually update the interpretation of the sequence, as well as changing the length of comps that were created at the same time.

This would be especially useful on when importing multipass EXR sequences with Create Composition turned on - this is a really useful feature that can create comps for all the passes, as well as a contact sheet and an assembly comp, but if it comes in at the wrong frame rate then both the source sequence and all the resulting comps need to have the frame rate and comp duration adjusted manually.

With something like RD Compsetter this isn't too bad, but when dealing with a lot of sequences being able to avoid it altogether by setting it at import time would be great for quality of life.

On a related note, I'd love to see an option for per-project import framerate settings - I've been caught out in the past when moving between projects on different frame rates and importing as 24fps into a 30fps project (for example), and only realising later once work has been done (Hernán Lindenbaum also mentioned this as an idea). @10440662 - please advise if this should be split off into a seperate request!

Cheers