Cleanup. (#132)

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

cmake/ftxui_export.cmake

@@ -0,0 +1,8 @@
+add_library(ftxui::screen ALIAS screen)
+add_library(ftxui::dom ALIAS dom)
+add_library(ftxui::component ALIAS component)
+export(
+  TARGETS screen dom component
+  NAMESPACE ftxui::
+  FILE ${PROJECT_BINARY_DIR}/ftxui-targets.cmake
+)