A3DGE Code

+ Added unit "a3dge.Common" with a few of useful pre-build game states.
* Modified a few examples to show ways to use the new Common game states.
* Improved the "Frame counter" object making it more intuitive.
* A small improvement in the TWorld3D constructor.
* A few documentation improvements.

* TOctree.CheckCollisions checks object radius and ignores if it's zero or less.
+ Added a few explanations in the examples headers and a few more aestetic
changes..
+ Added example ex_collision.

+ Added example of use of TWorld3D.

* Swapped exectution order of current and global states.
* Fixed TA3DGEApplication.SetTicksPerSecond: it didn't manage event
registration correctly.
+ Added a "frame counter" to the examples that shows the FPS rate.
+ Added comments to the ex_heightmap example.
* Some fixes in documentation and messages.