Previously the expected behaviour for front ends using the correct
API for hotlist, global history, cookie manager, and ssl cert
viewer was that the front end would initialise the treeview module
on startup and finalise it on application exit.
However, this meant that the front ends had to include the core
treeview header, which they didn't otherwise need.
Since the tree module provided access to the new treeview utilities
through the old tree API, and was used by front ends with no changes
for the new treeview API, the tree layer refcounted initialisations
of treeview-based widgets, and only called the underlying treeview
init/fini functions when needed.
This change moves that refcounting into the treeview module. Now
the hotlist, global history, cookie manager, and ssl cert viewer
widgets call call treeview init/fini as part of their own
initialisation and finalisation. This means that front ends
using the correct APIs for treeview-based widgets don't need to
know anything about the underlying treeview, and the tree module
compatibility layer has had its treeview refcounting removed.
Finally, the treeview_init function took a font size parameter.
Now it does not and lit gets font size from config. We probably
want to add a new `treeview_font_size` option to nsoptions, and
have differnent defaults on different platforms. 12pt on RISC OS,
and 11pt elsewhere, most likely.
