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.
Keypresses now go via content interface.
Contents don't shove the selection object into browser windows any more.
Contents report selection existence by sending message.
HTML content keeps track of where selections in it exist.
Contents report whether they have input focus via caret setting msg.
Caret can be hidden (can still input/paste) or removed.
Consolidate textarea selection handling.
Make textarea report its selection status changes to client.
Various textarea fixes.
Changed how we decide when to clear selections, and give focus.
