Removed vtkDeleteTreeFolder function.

Replaced all occurences in the code by vtkDeleteFolder
Added creation of .gitignore files in folder tree.

- In vtkFileSystemUtilities
	- added function vtkCreateTreeFolderWithGitIgnoreFiles
	- modified error management of vtkCreateTreeFolder

- In vtkMainFunctions
	- in vtkCreateProject, call vtkCreateTreeFoldeWithGitIgnoreFiles
	  instead  of vtkCreateTreeFolder

- In vtkCreateTreeFolderTester
	- added tests of vtkCreateTreeFolderWithGitIgnoreFiles
	- modified tests of vtkCreateTreeFolder so it fits with the new
	  error management.