Improved widget axis settings

We need to add more configuration options for the widget axes used by the histogram and scatter plot widgets:

• Ability to specify the unit
• Ability to specify a custom label
• Ability to specify the label "mode": if no custom label is defined, how many levels of the hierarchy should be displayed?
• The histogram x-axis title should be moved to the bottom and an y-axis should be added, as reported in #1697 (closed).
• Improve the UX for the statistics scaling:
  • Instead of showing the scaling option name, show a 'graph' icon that opens a dropdown.
  • Add a tooltip for each option that fully defines the scaling mathematically
  • Replace the fairly odd options (1/2, 1/4, 1/8) with simple log option?