Add ConfigurationListener, GlobalConfiguration

The LocalConfiguration already has a facility for listeners, add the
same for the Configuration held by the platform by introducing a
GlobalConfiguration interface.

Rename LocalConfigurationListener to just ConfigurationListener.
Refactor 'clipboard' setting to a global setting

The code was now implemented as a local listener whereas the setting
is actually global. Handling it inside SWTRegisterManager makes more
sense.

Also make sure to refer to the Options.CLIPBOARD instance, making
it easier to retrieve this code by using Eclipse's "Search Workspace
References" function.
Unnamed or named registers shouldn't use clipboard

Vim keeps its unnamed register around even when 'clipboard' is set to
'unnamed', it's just that it is no longer the default.

It is also kept around for when using named registers, as yanking to e.g.
"a will actually update "" in Vim while p will then paste the system
clipboard. Using either ""p or "ap the yanked content will be returned.

This commit makes Vrapper behave in the same way.