[patch] Panels integration

Problem/Motivation

One of our customer requested the integration of the SnapEngage Widget, luckily I came across this module :)
Unfortunately our customer has kind of complex requirements for displaying the widget.
The currently available global settings weren't flexible enough to fullfill this needes.
Thus I created a ctools content_type plugin to be able to use a SnapEngage Widget as a panels pane .
That way you can e.g. use different widgets on different pages.

Proposed resolution

Well, the attached patch :)
The approach is pretty straightforward. I've simply isolated the code to insert the JS and re-use it for the content type plugin.
Further I've changed the settings form a little bit - see User interface changes.

Remaining tasks

Reviews needed.

User interface changes

The widget id on the global configuration page isn't required anymore. This to make it possible to "disable" the global configuration in favour of the panels integration.
If the settings are saved with an empty widget id a warning is displayed to ensure the user is aware that the global widget is disabled.

API changes

none