Fix cmake plugin package for inline builds.

OS X only.

The PluginPackage file would only work for out-of-tree builds where only the plugin
was compiled. This enables in-tree builds with the rest of OpenCPN, hopefully
without breaking separate builds.
Ignore build files for git
Any data from radar triggers radar presence

We're not yet processing all message from the radars, so just assume
that any message is proof that it exists. This makes emulation of the
radar easier when using tcp dumps.

Also, improve error messages generated when trying to open UDP sockets.
Remove trailing spaces
Fix issue #56.

Set the label to a multi-line text on creation.