Includes changes from #17

• Remove the Delete Axis button from the Axes tab:
  • This includes removing the associated slot from the AxisTableMixin
• Add an inline "row delete" button to each row:
  • This inline button is the same as the inline row deletion button in the Traces tab, meaning that they look and behave the same way
• Change to DeleteRowDelegate:
  • Tooltip changed from "Delete Trace" to "Delete Row" to make it more general
• Change to ArchiverAxisModel.removeAtIndex:
  • If there is only one axis remaining, then deleting it would cause the plot to break
  • Instead, the model adds a new axis and removes the other one. This has the affect of resetting the only row and not breaking the plot
  • This captures an edge case