New clipboard API for front ends, and textarea progress

Text inputs and textareas used in HTML forms have never been robust and
we've decided to fix them for NetSurf 3.0.

Since the existing implementation is unmaintainable, the plan is to make
html forms use the desktop/textarea.{c|h} widget (which is currently used
for Hotlist node editing and the Atari URL bar).

The first stage of this is to add all the extra functionality we require
to the desktop/textarea widget, including clipboard support.

I've updated the front end clipboard API so now it can be used outside
browser windows, and to remove use of the html/text-plain selection
structure:

http://source.netsurf-browser.org/netsurf.git/commit/?id=e93fae7f2d9fdcc4ddf2a4db936cbd0dd33de20c

The Atari, Amiga, and BeOS front ends all need updated.

The RISC OS front end needs attention so that it can paste when another
application has ownership of the clipboard.

--

Michael Drake (tlsa) http://www.netsurf-browser.org/