Markdown: Another try at fixing the scroll jumping

When typing fast the webview would jump to the top, which was super
annoying. This is an attempt to fix the symptom but it doesn't really
address the real cause. Thanks to "moebiuseye" for the patch.
GeanyPy: Prevent exit() in console from exiting Geany

TODO: see about setting sys.excepthook to trap unhandled exceptions
in actual plugin code and rather than dumping to debug messages (or
exiting, in the case of SystemExit exception), present a dialog showing
a user-friendly message with option to see the details/traceback.