Add the ability to push and pop messages in to and out of the welcome

Add the ability to push and pop messages in to and out of the welcome
screen header, similar to the status bar. Hide the welcome body if the
message stack length is > 0.  Use this to display the "Waiting for
capture input data..." message when we're capturing from a pipe. This
lets the user know what's going on and keeps them from clicking on a
welcome screen item while we're waiting for data to arrive.  You can see
this in action by running

   (sleep 5; cat /path/to/a/capture) | wireshark -k -i -

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@28196 f5534014-38df-0310-8fa8-9805f1628bb7