Hmm, lasagne does not segfault anymore, after rebooting the device... Just calmly explains that camera lens cover is closed.
One point: I prefer apps written in native Gtk+Hildon, not Qt. Just personal taste.
What about making a barcode plugin for default camera?
And barcode-plugin-for-camera would in turn be extendable by plugins (like mbarcode is)?
About GUI: I would delegate clear-cell and delete-row somewhere closer to the table, not in a menu (what if user wants to do lots of clearning-deleting?), and move load-save-spreadsheet to standard top-menu. Then, the button to the right of the textbox would no longer be needed.
And, '+' button could switch to 'open lens cover to scan' screen which would save the trouble of clicking the button yet-again after opening lens cover (it should be possible to catch lens-cover-being-opened event and hence start scanning). Optionally, lens-cover-being-closed could switch back to spreadsheet.