Blender Git Loki

UI: Support tooltips for superimposed icons

In a couple of places in the UI, we show superimposed icons on buttons
to execute an operation (called "Extra Icons" internally). Hovering them
would show the tooltip of the underlying button, which is misleading and
confusing.
There are cases where it's not obvious what an icon does, so a tooltip
would be quite useful here. It's likely we are going to use superimposed
icons in more places in the future, e.g. see D11890.

The extra icon basically acts as an override for the button in the
tooltip code.

Differential Revision: https://developer.blender.org/D11894

Reviewed by: Campbell Barton