Automated image sequence offset calculation

Automated image sequence offset calculation for image input node and image editor.
    
Calculate offset or sequence when loading image in node or editor and also re-calculate offset when switching currently displaying image in editor or source image in input node. 

This makes using image sequences easier to use when they aren't starting from 1 (i've seen several times when artists used external tools to rename sequences instead of setting  offset for them)

Also this kind of recalculation is needed because offset is stored in ImageUser structure so when you're switching from one sequence to another it can be really annoying because offset from previous sequence would  be used for new sequence.