Extract common struct Dimensions from Terminal (#171)

- Convert Dimension to namespace to allow defining Fit method from dom.
- Use Dimensions extracted from Terminal as replacement struct.
- Convert Terminal to namespace as it only defines static members.
- Remove dom references from screen library (circular dependency).

src/ftxui/screen/terminal.cpp

@@ -18,7 +18,7 @@
 
 namespace ftxui {
 
-Terminal::Dimensions Terminal::Size() {
+Dimensions Terminal::Size() {
 #if defined(__EMSCRIPTEN__)
   return Dimensions{140, 43};
 #elif defined(_WIN32)