CMake modernization: add Backward::Interface and Backward::Object targets

[lahwaacz]

This PR should make it easier to use the backward.cpp file in downstream projects. As documented in the README, with this PR there are now three targets:

• Backward::Interface is an interface target that brings compiler definition
  flags, include directory, and external libraries. This is all you need to use
  the backward.hpp header library.
• Backward::Object brings Backward::Interface and backward.cpp as an
  OBJECT CMake library. This target cannot be exported, so it is not
  available when Backward is used via find_package.
• Backward::Backward brings Backward::Interface and backward.cpp as
  either STATIC or SHARED library (depending on the BACKWARD_SHARED
  option). This target is exported and always available, however note that the
  linker will not include unused objects from a static library, unless the
  -Wl,--whole-archive option (or similar) is used.

This PR marks the add_backward target as deprecated and removes the ${BACKWARD_ENABLE} variable.

Before this PR, using the Backward::Backward target was not easy and consistent:

• the BackwardConfig.cmake defined the target such that its interface had only the things needed to use backward.hpp (i.e. BACKWARD_INCLUDE_DIRS, BACKWARD_DEFINITIONS, BACKWARD_LIBRARIES)
• the main CMakeLists.txt file defined the target such that it contained even backward.cpp as either STATIC or SHARED library
• linking to a static library libbackward.a does not have the desired effect, unless the
  -Wl,--whole-archive option (or similar) is used (the linker discards unused objects from a static library)

[bombela]

Thank you for the fine work.