Added way to select and edit colors of placed Tris.
Added third_party catch unit test framework that builds the UnitTest in
Debug builds.
Refactor internal use of notification_text.
Rename src/imgui_helper.hpp to src/helpers.hpp
Added some helper functions, including "is_within_shape" (used for
selecting a Tri).
Fixed use of flags in helpers not using enum values.
Stephen Seo