QErrorMessage icons, check box, persistence, startup

QErrorMessage has been added to designer to display warnings about
syntax errors in the widget box files, plugins not loading, etc. Some issues have crept up:

• It should be possible to retrieve/set the messages the user refused to make the behaviour persistent over consecutive app starts (and probably offer a context menu 'Clear all refused msgs'). This is really important for Designer, please
  let the Tools Team know when it is done.
• Duplicated messages that are already queued are not suppressed if the user unchecks them
• it should be possible to customize the icon (according to msg severity)
• it should be possible to customize the msg types displayed by qtHandler . For example, we might not want debug msgs at all.
• Windows users noted that the checkbox should be inverted (do not show...)... Maybe a policy should be added.

• Very nice to have: Some clever behaviour to display app startup errors. Currently, what happens is that
  if QErrorMessage pops up before the main window is visible, it will hidden by the main window. This requires users to somehow cache
  startup msgs until the main window becomes visible. Some magic to prevent this would be nice.

• Also very nice to have: Optionally match by message format string, for example
  showMessage(tr("Accept cookie from %1?"), "www.hackers.com");