Beginning of cef3 work.

Code compiles cleanly, runs and loads pages. wxWebView API still to
be implemented. Most of the cef1 work removed as a clean start is
needed.
Implement window sizing.

Start work on the client handler as it is required for the sizing.
Add a copy of the wxWidgets sample.

This will allow better testing of more advanced features. Also
reorganise samples folder.
Add support for LoadURL, Reload and Stop.

Start implementing a client handler as it is required for almost
all functionality.
Expand the simple sample.

Adding a button allows the quick testing of new features.
Add support for Cut, Copy, Paste, Undo, Redo and SelectAll.

Also add a missing call to event.Skip in OnSize.
Simplify IsBusy logic.
Add support for SetZoom, RunScript, GetCurrentURL and DoSetPage.
Add GetTitle and title event support.
Add history support.
Add default zoom value.

This needs to handled properly in the future, but for now always
return a medium zoom level.
Make sure all paths return a value in IsBusy.
Implement GetNativeBackend
Add error handling
Merge branch 'cef3'
Merge cef3 work into main branch.

Detect the version of cef being included and choose the correct
implementation.