Cleanup. (#132)

- Split the large CMakeList.
- Remove travis
- Remove gitlab-ci.yml

cmake/ftxui_benchmark.cmake

@@ -0,0 +1,20 @@
+if (NOT WIN32)
+  FetchContent_Declare(googlebenchmark
+    GIT_REPOSITORY "https://github.com/google/benchmark"
+    GIT_TAG 62937f91b5c763a8e119d0c20c67b87bde8eff1c 
+  )
+  FetchContent_MakeAvailable(googlebenchmark)
+
+  add_executable(ftxui_benchmark
+    src/ftxui/dom/benchmark_test.cpp
+  )
+  target_link_libraries(ftxui_benchmark
+    PRIVATE dom
+    PRIVATE benchmark::benchmark
+    PRIVATE benchmark::benchmark_main
+  )
+  target_include_directories(ftxui_benchmark
+    PRIVATE src
+  )
+  set_property(TARGET ftxui_benchmark PROPERTY CXX_STANDARD 17)
+endif()