dtkComposer requires dtkCoreSupport

Created by: jwintz

When building dtkComposer layer, if dtkCoreSupport is not enabled, the compilation fails as follows:

[ 50%] Building CXX object src/dtkComposer/CMakeFiles/dtkComposer.dir/dtkComposerViewLayoutItem.cpp.o
In file included from /Users/jwintz/Development/dtk/src/dtkComposer/dtkComposerViewLayoutItem.cpp:20:
In file included from /Users/jwintz/Development/dtk/src/dtkCoreSupport/dtkAbstractView.h:23:
/Users/jwintz/Development/dtk/src/dtkCoreSupport/dtkAbstractObject.h:23:10: fatal error: 'dtkCoreSupportExport.h' file
      not found
#include "dtkCoreSupportExport.h"
         ^
1 error generated.

No more reference to dtkAbstractView should remain in non support layers.