MyCyma

[fix] ensure ArtworkStep has a non-nil/NULL 'position' (not set in ArtworkStepThumbnail for example)

[evol] added helper methods for ArtworkStep<Image|Thumbnail>

[fix] tries to regen missing thumbnail

[cosmetic] added position number to artwork_step_images, only visible in the admin pages, to be able to change order for steps made the same day (closes #16)

[cosmetic] use first artwork image instead of last for set thumbnail in the tree view (closes #17)

[cosmetic] properly align images vertically in an image list

[cosmetic] display groups in a set into a light blue cell, like in the tree (closes #18), and btw improved CSS handling a bit (unfinished)

[cosmetic] invert arwork list and group list in set view (refs #18)

[cosmetic] improved front view (closes #13)

[cosmetic] artwork_sizes height and width are now integers in the DB, and displayed with the corresponding unit (closes #14)