Provide contexts for a 'Welcome' string. Sometimes we're saying hello, other...

Provide contexts for a 'Welcome' string. Sometimes we're saying hello, other times referring to the welcome panel. props pavelevap, fixes #19698.

git-svn-id: https://core.svn.wordpress.org/trunk@19807 1a063a9b-81f0-0310-95a4-ce76da25c4cd