Added frame offset options

XDTS expects frames to start from 0 but many apps such as OpenToonz and Toei Animation Digital Exposure Sheet start at 1. This results in a 1 frame offset from the number in the CSV file and what it actually ends up as when the converted XDTS is imported into said apps. For example, a frame of 5 in the CSV will end up as 6.

Frame Offset can be used to address this, the default of -1 will result in the numbers matching.

An in-app help button is present to explain this.