Using mathmlCreateNode function

Replace some duplicated code with call to mathmlCreateNode.
Replace mathcolor with background-color

With background-color is easy to say of with element the mouse
is over.

The choice for orange as the background-color was because of a good
contrast with the white background and the back font.

Note: Look like that margin and padding doesn't effect mathml
elements. And border don't have a uniform behaviour.
Initial support for drag and drop

* Add first support for drag and drop (across equations).
* Change mathmlSetup to cover drag and drop.
* Add simple example of drag and drop.
All events start with a single click

* Refactor the code so that all events start with a single click.
* Remove some jQuery call when not need.
* Remove partially duplicated function to replace mi element with mn.