Updates to Python GTK+ 3 Tutorial

I continued working on the Python GTK+ 3 Tutorial since I announced it almost a month ago. I added a section describing the Gtk.Grid widget, menus, dialogs and Gtk.TextView, the latter just added today. In addition, I added screenshots for all examples and merged a couple of grammar/typo fixes from other people. The current contents should cover the most important widgets and should allow you to create more complex applications. Still missing are some advanced topics such as Drag-n-Drop and creating custom widgets such as CellRenderes or GObject sub-classes. Although, I tried to describe all classes and methods I used in the examples, it’s still sad that there is currently no replacement for the excellent PyGTK reference. Because of that I decided to include a stripped down version of the API reference in the tutorial. Hopefully, people don’t get confused by this and expect it to reflect the complete API. The source of the tutorial and all examples are available at my github repository.