Create Tkinter example with navigation bar and tabs

[cztomczak]

There are focus issues when switching between url entry <> browser. To fix this CefFocusHandler will need to be implemented.

Focus issues continued in Issue #284.

[cztomczak]

Url entry widget focus issues fixed in b80ff78. However there is still one issue, url widget loses keyboard focus when mouse hovers over CEF browser. For more details on the focus issues see this topic in the upstream CEF Forum: Native widgets don't receive keyboard input.

Tabs are still todo.

[cztomczak]

CEF focus issues reported in upstream Issue #2026.

[cztomczak]

Tabs won't be implemented in Tkinter example. There is already a navigation bar and that's enough I think. Tkinter is not an easy to use toolkit, in Qt/wx things get done with x2 less code. Also tkinter crashes on Mac (#309) with no way of debugging it. Investing more time to implement new features doesn't seem like a good idea. Tabs should be implemented in wxPython, that's the most stable and most widely used framework from my experience.

[cztomczak]

Focus issues continued in Issue #284.