Fix ButtonClusters warnings

[finetjul]

Visual Studio warnings:

2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(512) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(514) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(558) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(623) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(634) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(638) : warning C4267: '=' : conversion from 'size_t' to 'int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(747) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(776) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(805) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(806) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(830) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data
2>..\..\..\..\..\MSVTK\Libs\VTK\Widgets\msvVTKWidgetClusters.cxx(831) : warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', possible loss of data

13>..\..\..\..\MSVTK\Applications\ButtonClusters\msvQButtonClustersMainWindow.cxx(266) : warning C4267: 'initializing' : conversion from 'size_t' to 'int', possible loss of data