[Feature] Have drag lines/points rendered above heat series data

**Is your feature request related to a problem? Please describe.**
I'd like to add a cursor made up of a pair of drag_lines and a drag_point on top of a heat series plot for moving around and investigating different aspects of the data. However it seems like there's no way to get the dragable objects to show up in front of the data, and instead they are hidden by the data, albeit still functional.

**Describe the solution you'd like**
It seems like the drag lines should be able to show up in front of the plot, but by default they are put before the axis/heat_series in the render order, and there's no way to set the axis objects to render before the dragables, as there's no `before` keyword for `add_plot_axis()`. So, it seems like having this option to set the axis and plot series data to be set to render before the dragables would be ideal.

**Describe alternatives you've considered**
I've tried playing around with staging, and manually changing the relevant items ordering in their parents without much success.

**Additional context**
Here's a simple example of how the items appear below the data. Despite being added after.