A couple of small visual improvements for lui

Change titles to white rather than green text to improve readability on blue
background, and use erase() instead of clear() to reduce flicker in the source
window.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192768 91177308-0d34-0410-b5e6-96231b3b80d8