Add "don't show this again" option for automatically opened Project Upgrade view

[dgrunwald]

SD-1821, originally created on 4/5/2011 23:01:42 by Daniel Grunwald

It's useful to have the ProjectUpgradeView pop up automatically, since
the user might not be aware that upgrading to SharpDevelop 4.x does not
automatically make his projects use C# 4.0 / .NET 4.0.
However, if the user wants to continue using an old C# version, the
upgrade view should not appear every time the project is opened.

Implement one of these options:

1. "Do not open this view automatically" checkbox that globally turns
   off the automatic opening of the UpgradeProjectView.
2. Show the view only once per project.