Support tooltips for ui.text() / markdown elements

**Background**

The insights team is currently using `...` to display tooltips. These are OS tooltips, and don't support rich content or improved usability like that of the Fabric [callout](https://developer.microsoft.com/en-us/fabric#/controls/web/callout)

**Goal**
Devise a mechanism that allows portions of content withing the body of `ui.text()` to be annotated with tooltip content.

Related to h2oai/q#637

**Notes**
Let's discuss alternatives/approaches before this is implemented. The tricky bit here is that `ui.text()` accepts markdown, and the body of the markdown needs to support multiple annotations. Preferably, let's not use HTML annotations because HTML is annoying.