item-data.c: fix identation
item-data.c: make doc more explicit
sheet-item.c: fix indentation, remove trailing whitespace
SheetItem: Remove most of ghost/floating items offset on rotation, closes #83.

Also introduces FIXME_* defines.
Sheet: Use `grid_item` instead of `grid`, fixes #96.

Also add some minor amounts of documentation.
Sheet: Extend arrow-key handling.

Arrow keys now can move the selection when unmodified, or - if modified by <kbd>control</kbd> - scrolls the sheet.

Bug: Currently the focus jumps back to the part selection pane.
Bug: The `SheetItem` does not update, the `ItemData` does though.