Qt console: Fix _prompt_pos accounting on timer flush output.

[pankajp]

When output is appended to the frontend widget by the output
flushing timer, the _prompt_pos was not being modified accordingly.
This could cause incorrect prompt position resulting in apparent
freezing of the prompt in some circumstances.

Running the following file and then doing p.edit_traits() a couple
of times in the frontend and clicking the button shows the error
some times, when the prompt stops responding to enter and
backspace once the prompt pos exceeds the document's size.

from traits.api import *
from traitsui.api import *

class Problem(HasTraits):
  e = Button

  def _e_changed(self, content):
    print 'E CHANGED', content

  traits_view = View(
    Item('e'),
  )

p = Problem()
p.edit_traits()

I would also like this to be backported into 1.x branch, but i'm not sure of the procedure.

[takluyver]

I see you've also moved this call earlier within _show_prompt. Do you want to comment on that?

[pankajp]

Oh yes, now that i think of it, the pending output should obviously be flushed before writing the new prompt. Not sure whether that needs any further explanation.

[takluyver]

Thanks, we've discussed it in the office at Berkeley. We'll merge it, but we don't feel we understand that bit of the code well enough to be confident that it won't introduce other problems, so we're not going to backport it to 1.x at the moment.