use display instead of send_figure in inline backend hooks

This lets custom display functions (e.g. HTML) be used without any extra changes (see #2234).

possible downsides:

* The previous code guarantees that only one format is published. If multiple figure formatters are registered, display will send them all.
* If people for some reason disable the type-printers, then they will display the automatic figure display.

Neither of these cases can come up unless people are messing with the formatters, and I think the first is actually an improvement.

IPython/zmq/pylab/backend_inline.py

@@ -15,6 +15,7 @@
 
 # Local imports.
 from IPython.config.configurable import SingletonConfigurable
+from IPython.core.display import display
 from IPython.core.displaypub import publish_display_data
 from IPython.core.pylabtools import print_figure, select_figure_format
 from IPython.utils.traitlets import Dict, Instance, CaselessStrEnum, CBool
@@ -102,7 +103,7 @@ def show(close=None):
         close = InlineBackend.instance().close_figures
     try:
         for figure_manager in Gcf.get_all_fig_managers():
-            send_figure(figure_manager.canvas.figure)
+            display(figure_manager.canvas.figure)
     finally:
         show._to_draw = []
         if close:
@@ -138,7 +139,7 @@ def draw_if_interactive():
 
     if not hasattr(fig, 'show'):
         # Queue up `fig` for display
-        fig.show = lambda *a: send_figure(fig)
+        fig.show = lambda *a: display(fig)
 
     # If matplotlib was manually set to non-interactive mode, this function
     # should be a no-op (otherwise we'll generate duplicate plots, since a user
@@ -191,7 +192,7 @@ def flush_figures():
         active = set([fm.canvas.figure for fm in Gcf.get_all_fig_managers()])
         for fig in [ fig for fig in show._to_draw if fig in active ]:
             try:
-                send_figure(fig)
+                display(fig)
             except Exception as e:
                 # safely show traceback if in IPython, else raise
                 try: