Widgets (bg, plug, new), Load cover, Blog entries

* Placeholder backgrounds; new implementation that controls the
background styling of placeholders.
* Pluggability widgets; plugins can/will now overrule widgets from
framework if they 'share' the same name
* New widgetds; logo, signpost, social, callout, bio (replicated from
playground plugin)
* Load cover; screen is displayed after CSS rules are applied,
preventing styling issues
* Detection of images is extended to also be triggered by meta data
'image' (next to 'imageid' and 'thumbid')
* Blog entries now has social feature (Twitter, FB Like); first attempt,
will be enhanced further