[PATCH] Backend notifiers.

[Maciek Gajewski]

Hello

Assigned patch adds backend notifiers to graphics code. The patch is 
cumulative, contains previously presented locking facilities.

I ask You for review, and - eventually - inclusion into main source tree.

How does it work:

Graphics backend implements three methods:

virtual void property_changed (const graphics_handle&, const std::string&);
virtual void object_created (const graphics_handle&);
virtual void object_destroyed (const graphics_handle&);
(stubs added to gnuplot and fltk backend)


- property_changed is called from base_property::set
- object_destroyed is called from gh_manager::do_free
- object_created is called from gh_manager::do_make_graphics_handle
  and gh_manager::do_make_figure_handle.

I've been using this code for last few days with my Qt backend, and it works 
good, events are going back and forth between threads, UI is responsive and 
changes are visible immediately. You can event do animations!

For instance, this code:

 x = [-pi:0.2:pi];
 l = plot( x, sin(x))
 for s = [0:0.1:100]; set(l,'ydata',sin(x+s)); usleep(10000);endfor

creates figure window with sine wave moving slowly like signal on oscilloscope 
screen. Figure remains responsive, can be moved and resized, while the wave 
still moves.

Maciek Gajewski



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