Example change

In openSCAD application, we have examples named like Example001.scad, Example002.scad etc. With such name, the user can't interpret the purpose and functionality of a particular example. If he needs a certain type of example code, he has to open every example to see its code and guess its functionality.
In this patch, I categorized the present examples into four categories - Basics, Extrusion, Shapes and Advanced and also provide the appropriate names to .scad files. 
Also I changed the approach of accessing the Examples in the application a little. Now, in file menu, When you'll click 'Examples' option, a dialog box will open in front of you having the categories of examples, you can select any category and open the .scad file inside that category having easily understandable names, according to your requirement. I preferred this approach because, in future, when we'll add more examples, it won't increase the size of the sub-menus of example menus. So, we can add as many suitable examples as we want to give better experience of learning to the user. :)