Better data label customization

When data labels are enabled on visualization types where data overlaps (such as clustered bar charts and line charts), there are occasions where data labels do not display, or it is difficult to tell which label corresponds to which bar/line. There should be better customization options for Data Labels, which would ease comprehension of various visualizations. In particular, for the varied types of bar charts:

- Label location: start of bar, middle of bar, end of bar, end of chart
- Label/Background colour matching: change the label's text or background to match the bar to which it corresponds
- Label Stacking? If a label would overlap with another, display both of them slightly offset, instead of only displaying one label.