Created new plugin richContent.js on the basis of content.js

* Rich-text formatting works for the main styles: bold, underline,
strike, italics.

* The selection through double click does not work. The selected range
object is different in that case. Must find a way to adapt it before
editing.

* The blur event cannot be used as-is to close the plugin. The plugin
must close only when the user clicks outside of the button box...