Add Browser.Free() method for freeing CEF references, as "del browser" is not guaranteed to do so

[cztomczak]

The del browser statement only removes a reference to the object and does not guarantee that object is destroyed. Thus PyBrowser.cefBrowser C++ object reference is not freed.

Add also Frame.Free() and maybe for other objects as well.

Check __dealloc__ methods if freeing resources there should be done in the Free method instead.

Related: upstream Issue #2593: Improvements for debugging CefRefPtr lifespan issues.

Update documentation and examples.

[cztomczak]

To check if CEF references were freed properly you have to run in DEBUG mode. See the upstream issue that shows how to debug CefRefPtr lifespan issues in CEF v73+: https://bitbucket.org/chromiumembedded/cef/issues/2593/improvements-for-debugging-cefrefptr