The attached patch adds support to CEF3 cefpython for the GetSource() and
GetText() methods. These methods are asynchronous in CEF3 unlike in CEF1.
StringVisitor C++/Python classes are added and used to call into a Visit()
method to cef to be asynchronously called when it has the page source or text
ready to be returned.
This is used to pass the returned source or text into a user's python program.
wxpthon.py and wxpython.html have been modified to demonstrate both methods
with javascript calls to dump page source or page text to console.