Gen popup impl. #18

* Added pagetemplate context processor
* Added the "gotosheet" optiontype and the "custom" optiontype
* Optiontypes now have a "persistbehaviour" that determines whether
options are persisted or not. When set to "readonly" they will not be
stored. If set to "writeid" the id (plus values) will be stored. Can be
extended in the future to support storing optiontypes that are
"compound".
* Pagetemplate for "Page" type now has a generic popup window that
enables users to pick the header, subheader, sidebar, subfooter and
footer.