Add an API for subtracting UI elements.

Status Job ID Name Coverage
  External
passed cdash

passed ghostflow-branch-check-master

failed smtk@master-kerbin-windows-shared-release+examples+remus

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+private+remus

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+python3+remus+vtk

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus+vtk

passed smtk@master-norkit-osx-shared-release+examples+matplotlib

running smtk@master-praxis-windows-shared-release

825:58:40

failed smtk@master-talosiv-osx-shared-release+examples+matplotlib

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib+private

passed cdash

failed smtk@master-kerbin-windows-shared-release+examples+remus

00:01:22

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+private+remus

00:03:17

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+python3+remus+vtk

00:00:06

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus

00:03:18

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus+vtk

00:03:10

passed smtk@master-norkit-osx-shared-release+examples+matplotlib

00:01:06

failed smtk@master-talosiv-osx-shared-release+examples+matplotlib

00:06:40

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib

00:01:50

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib+private

00:02:13