CMake: Proper use of TREE argument in source_group

QtCreator should not create a top level virtual folder named TREE when using source_group(TREE <src>) command.

The reasoning for this is, as per CMake specs, the TREE option is a special argument for the source_group command, it is not the actual <name> argument, and as such it should be considered a distinct case, i.e. "CMake will automatically detect, from <src> files paths, source groups it needs to create, to keep structure of source groups analogically to the actual files and directories structure in the project. Paths of <src> files will be cut to be relative to <root>".

So in my opinion the actual fix would be simple: when source_group(TREE <src>) command is used, do not consider TREE as a group <name> argument but treat it as a special case (as per spec) and don't create a virtual folder named TREE (just use the the physical file organization from the file system, and place the top directories relative to the <root> as direct children of the project node).

As an example, when using source_group(TREE ${PROJECT_SOURCE_DIR}) the project structure:

1. should look like this (GOOD):

<target>
   | actual_dir_on_fs
      | src_file.h
      | src_file.c
   | another_actual_dir_on_fs
      | another_src_file.h
      | another_src_file.c

2. instead of (BAD):

<target>
   | TREE
      | actual_dir_on_fs
         | src_file.h
         | src_file.c
      | another_actual_dir_on_fs
         | another_src_file.h
         | another_src_file.c

3. or instead of (BAD):

<target>
   | Header Files
      | actual_dir_on_fs
         | src_file.h
      | another_actual_dir_on_fs
         | another_src_file.h
   | Source Files
      | actual_dir_on_fs
         | src_file.c
      | another_actual_dir_on_fs
         | another_src_file.c

Applying the attached patch seems to fix the problem, and the project structure looks like case 1 above(0001-CMake-Proper-use-of-TREE-argument-in-source_group.patch).

 Related issues: QTCREATORBUG-23372 and QTCREATORBUG-23318.