Separator ftxui::{screen,dom,component} into separate build unit.

examples/dom/CMakeLists.txt

@@ -1,3 +1,8 @@
+function(example name)
+  add_executable(${name} ${name}.cpp)
+  target_link_libraries(${name} PUBLIC dom)
+endfunction(example)
+
 example(blink)
 example(bold)
 example(color)