Fix compilation error in ctkDICOMThumbnailListWidgetTest1
Merge branch '272-thumbnails-hidden-in-dicom-browser'

* 272-thumbnails-hidden-in-dicom-browser:
  Fix compilation error in ctkDICOMThumbnailListWidgetTest1
Add ctkProxyStyle and apply to ctkCheckBox, ctkCollapsibleGroupBox

Closes #262, #285
Fix ctkCheckBox style: variable case
Merge branch '285-ctkCheckBox-style-conflict'

* 285-ctkCheckBox-style-conflict:
  Fix ctkCheckBox style: variable case
  Add ctkProxyStyle and apply to ctkCheckBox, ctkCollapsibleGroupBox
  Fix compilation error in ctkDICOMThumbnailListWidgetTest1

Conflicts:
	Libs/DICOM/Widgets/Testing/Cpp/ctkDICOMThumbnailListWidgetTest1.cpp
Cleanup ctkProxyStyle unused variable/code
Merge branch '285-ctkCheckBox-style-conflict'

* 285-ctkCheckBox-style-conflict:
  Cleanup ctkProxyStyle unused variable/code