Stronger support for popovers

[damithc]

This is what I'm trying to achieve. The current popover needs some enhancements before we can achieve this level.

On the code side, something like this would be ideal but not sure how close we can get.

These solutions are collectively known as _design patterns_, a term popularized 
by the seminal book *Design Patterns: Elements of Reusable Object-Oriented Software* 
the so-called {{“Gang of Four”}}(gof-tip) (GoF): Eric Gamma, Richard Helm, 
Ralph Johnson and John Vlissides. 

Somewhere else in the file ...

<popover-content id="gof-tip">
<pic src="introduction/book-designpatterns.jpg" width="400px">
Lorem ipsum dolor sit ...
</popover-content>

OR

<popover-content id="gof-tip" src="gof-tip.md"/>

[Gisonrg]

trigger should fix the issue now, though we haven't implement the markdown syntax for it.