Associate DCMTK_DEFINITIONS to CTKDICOMCore

Following commit 12c052ff1, FindDCMTK.cmake has been updated to
detect if HAVE_CONFIG_H should be passed when compiling against
older version of DCMTK packaged on Debian.

To keep build system of project depending on CTK simple, instead of
manually adding the definition for all target depending
on CTKDCMTKCore, the definition is added as a target
property INTERFACE_COMPILE_DEFINITIONS to CTKDCMTKCore in CTKConfig.cmake.

That way, all target depending on the imported target CTKDCMTKCore
will automatically specify the -DHAVE_CONFIG_H flag.

Since the property INTERFACE_COMPILE_DEFINITIONS has been added
in CMake 2.8.11, it will still be required to specify the definition
manually in project using older version of CMake.

See #413
Merge branch '413-associated-dcmtk-definitions-to-ctkdicomcore'

* 413-associated-dcmtk-definitions-to-ctkdicomcore:
  Associate DCMTK_DEFINITIONS to CTKDICOMCore

Fixes #413
Active window flag transferred on RequestSoftwareInputPanel event

The widgets on a ctkPopupWidget could not get the keyboard focus
because the window of the popup widget was not active. Since only
one window can be active at a time, the 'active' flag should always
be transfered to the window that requests the keyboard input.

Fixes #409
Merge pull request #410 from NifTK/409-widgets-on-ctkPopupWidget-do-not-get-keyboard-focus

Active window flag transferred on RequestSoftwareInputPanel event
Add DCMTK 3.6.0 support to CTK application.

Since the ctkDICOMUtil method have been updated in commit 12c052f to work
with DCMTK 3.6.0, this commit update the application code to use it.

See #405
Merge branch '405-support-dcmtk-360'

* 405-support-dcmtk-360:
  Add DCMTK 3.6.0 support to CTK application.
Add convenience function "ctk_list_to_string"

See #418
Automatically reformat list passed to "CTK" inner build.

See #418
COMP: Add support for CTK_USE_SYSTEM_* option

<<< WORK IN PROGRESS >>

This commit update CTK build system to support CTK_USE_SYSTEM_* variable.

The new macro "ctkCheckExternalProjectDependency" allows to recursively
traverse the external project dependency tree to add the required projects.
It also print an informative message ...

Preliminary support for:

  -DCTK_USE_SYSTEM_ITK:BOOL=ON
  -DCTK_USE_SYSTEM_DCMTK:BOOL=ON
  -DCTK_USE_SYSTEM_OpenIGTLink:BOOL=ON
  -DCTK_USE_SYSTEM_VTK:BOOL=ON

See #418