Cleanup. (#132)

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

cmake/ftxui_git_version.cmake

@@ -0,0 +1,13 @@
+find_package(Git QUIET)
+if (Git_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git")
+  message("git found")
+  execute_process(
+    COMMAND ${GIT_EXECUTABLE} rev-list --count HEAD
+    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+    OUTPUT_VARIABLE git_version
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+  )
+else()
+  set(git_version 0)
+endif()
+