Add Option to Allow Tab/Back-tab list item indent to treat empty items differently

[vsch]

• Add: MdTabOverride/MdBackTabOverride option to treat empty items differently:
  • treat empty items as having no children, should only indent/unindent the item in all cases.
  • should roll-over between highest and lowest indentation level when the item can no longer be indented to allow tab to be used to adjust sub-item to parent item

This allows using repeated tab key clicks to adjust newly inserted list item for all possible indentation levels without needing to use Shift+Tab. Similarly for back-tab.