Refactor internal resource location APIs and initialization

Rename cmSystemTools::FindExecutableDirectory to FindCMakeResources.
Teach it to compute the locations of cmake, ctest, cpack, ccmake, and
cmake-gui executables, and the location of CMAKE_ROOT.  Provide this
information from static cmSystemTools::Get<resource>() methods.
Refactor code that needs these locations to use the new APIs.

Teach FindCMakeResources to use the OS X system API to lookup the
executable location.  When running from the CMake build tree itself,
leave a file in the tree that FindCMakeResources can use to read the
location of the source tree.  This avoids the need to compile the source
tree location into a binary that may be installed and used without the
source tree.

Teach the QtDialog on OS X to create a "cmake-gui" symlink in the build
tree next to "cmake" and the other tools, as is already done in the
install tree for the application bundle.  This ensures a consistent set
of executables are available in one directory.