Sheet: Add comments, remove trailing whitespace.
item-data.c: fix identation
item-data.c: make doc more explicit
sheet-item.c: fix indentation, remove trailing whitespace
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.
PartItem: Simplify bounds caching.
SheetItem: Properly translate/rotate floating/ghost items.
Sheet: Remove old debug message.
test: Add a always failing test.

This shall only be used when debugging the buildsystem. It is also vital for Travis CI tests.