Qt console: Fix _prompt_pos accounting on timer flush output.

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.
Merge pull request #4768 from pankajp/fix-qt-prompt-pos-accounting

Qt console: Fix _prompt_pos accounting on timer flush output.