Some float properties don't export to fbx

Export fbx currently exports specific properties that match onto existing maya attributes. Some properties don't match directly but they should still be exported as "custom attributes".

1. Drag in a point light, add it to sequencer
2. Animate the "Source Length" property
3. Export to fbx
4. Import to maya, look at the point light extra attributes - there should be a Source Length attribute in maya with those keyframes