Message dialog fixes

Here are the `OCPNMessageBox` fixes I talked about on the forum.

- The first commit, 2902368, makes the biggest change; it uses native message dialogs where possible, to avoid all the issues we have with our custom dialogs.

- 63a8422 prevents the unwanted chart interactions while message dialogs are visible.

The others are some minor things I came across while working on these dialogs:

- da6c103 enables some dialogs which are needlessly disabled on OS X.

- 26cc270 removes cancel buttons from some dialogs which shouldn't have them - plain yes/no dialogs where cancel makes no sense. (Note that some dialogs remain where cancel does make sense, and in those I have left it.)

- bbac728 fixes an error I came across which causes the app to freeze if a route paste action is cancelled.

I hope you do not consider these fixes too major to commit now, as I feel it would be nice to fix the current dialog issues and so the first two commits are important.
But if you do decide to defer these fixes until 4.2 I will understand...

Caesar