Added content interface for search.
Removed bw->cur_search search context. Desktop layer now does nothing
except pass search requests from front end onto the bw's current_content
via the content interface.
Search API reduced to a pair of functions at each level:
{desktop|content|html|textplain}_search
and
{desktop|content|html|textplain}_search_clear
Updated front ends to use simplified search API. Only tested GTK and RO builds.
These confine the search stuff to render/. However search still uses struct
selection. The handling for which is still spread over desktop/ and render/.
Also the render/search code itself still fiddles inside html and textplain
privates.
