Implementing caching for widgets.

#6043 
* [x] Custom html
* [ ] Taxonomy
* [ ] Featured posts
* [ ] Feed reader
* [x] Publications
* [ ] Slideshow (be ware - this might need assets)
* [x] Embed media (be ware - this might need assets)
* [ ] Twitter
* [ ] Tabs (be ware - this might need assets)
* [ ] Mini calendar
* [ ] Recent presentations
* [ ] Recent FAQ's
* [x] Accordion
* [x] Facetapi taxonomy
* [x] Manual lists
* [ ] Site logo
* [x] List of (`os_sv_list`).