Detects if the current request is declared as 'pure', i.e. affects
no widgets or internal application state, but merely is a request for
information. Such requests simply return the result of the function
that represents the action and are used by some AJAX operations to
retreive information (suggest block, etc). When such requests are
satisfied, the actions have access to the session, the widgets, and
all other parameters. However, none of the callbacks (see
*on-pre-request*) are executed, no widgets are sent to the client,
etc.