This callback is useful to broadcast the last print line sent in the form of
the parsed gcoder.Line object. We can then augment the gcoder.Line to do
awesome stuff, such as updating the print status in the 3D viewer.

This might speed up Pronterface while printing.
Indeed, as we keep adding items we keep expanding the list and Python has to
move it all the time.

I'm hunting unjustified try: except: blocks.

The icon was adapted from the 24x24 "Refresh" icon from Humanity Icon theme.

As far as I understand, a wxFrame needs a wxPanel child, but we shouldn't
reparent this wxPanel to some other container. As a consequence, I created an
initial wxPanel for the gwindow wxFrame and assigned it the previous sizer
(instead of using the gviz panel as the wxFrame's panel).
Not sure why it wasn't segfaulting on my other systems, though.

Not sure about that dropping of the self.gwindow.Destroy try block, we'll see

There is a lot of code duplication between addgcode() and add_parsed_gcode,
which would probably need to be cleaned up.

This might be slightly off, but it could also be the zooming code that's off.

This commit mostly removes unused code and cleans some things up.
The main contribution is that instead of calling repaint() and Refresh(), we
now only mark the canvas as dirty and call Refresh(), which will do the repaint
just-in-time. This speeds things a lot when quickly changing layers or zooming,
as it does not repaint once per keyboard/mouse event but once per actual
Refresh, which makes it much smoother.

FWIW, on my test system with my test GCode loading time goes down from 5.7s to
2.3s between b3a673bc and this commit, that's almost 60% gone.
However, there is still a lot of room for functional improvements :
- I don't think the roundtrip through the target array is useful
- There's no handling of relative coordinates (neither for position nor for e)
- The arcs drawing code might be wrong
- The stlview bits are not enabled and probably have to be looked at and updated