SilvaExternalSources can not be disabled

Is this even desired? Locally, I've altered kupu so it displays a message if ES isn't in the installed extensions list. I can do something similar for widgets, but that would be after the source element is already added (you can't conditionally hide a widget, so it's always available).

Consider this:
An import of a publication is done into a fresh silva without external sources installed. A document in the import contains an external source. Should the source in the document be able to be removed or edited (in kupu?) It'll always be removable in kupu. Verify that existing ES's _can_ be edited / removed if ES is not installed . Perhaps also display a note in kupu tool and widget informing editor that ES is not installed.

Note that we are now distributing External Sources in the 'all' tarball, it's not in silva-extra anymore.

However, it could be that some user doesn't want it available so ES shouldn't become a dependency. I agree that it must be possible to edit a doc with an external or code source included and remove it, even if the ES product isn't installed.

When ES is not installed:
1) in kupu: new sources cannot be added, existing sources can be edited, message displayed in Tool when not editing an existing ES about the tool being disabled as it's not installed.
2) In Forms editor, existing es's can be edited, when a new e.s. is inserted, a message appears in place of the "select source" box informing that ES is disabled.
When ES is installed:
The system works normally!

I've put a number of "not high pri" fixes into the 2.0 branch, for 2.0.1. This once is _really_ not that important, so I've put it in the trunk only (2.1)