notify page of Alpheios activated status

[balmas]

moved from alpheios-project/components#326

requested by PHI -- for sites that want to change their behavior if the user has the Alpheios extension installed, it would be useful if there was a reliable way to check whether or not the Alpheios tools are enabled or not, such as the use of a data attribute on the Alpheios popup or on the document body.

I think the better way to handle this is via an event, similar to the Alpheios_Embedded_Check. Setting an attribute isn't reliable because it can change many times in a single page load.