Add the Catch2 headers to third_party/include/catch2.
Add Catch.cmake and CatchAddTests.cmake to cmake/.
Add unit tests src/wx/tests/strutils.cpp using Catch2 to test
src/wx/strutils.cpp.
Make some code changes to strutils.cpp to make the tests pass.
See src/wx/tests/CMakeLists.txt for how to set up unit test files; they
plug into the normal CTest mechanism in cmake.
The test binaries are written to the tests/ subdirectory of the build
directory.
Building the tests and enabling the CTest support can be turned off by
passing -DBUILD_TESTING=OFF to cmake, the default is ON, this is the
standard mechanism.
Start running ctest on travis and appveyor.
Move stb to third_party/include where Catch2 is now as well.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>