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).

include/ftxui/screen/terminal.hpp

@@ -2,24 +2,23 @@
 #define FTXUI_CORE_TERMINAL_HPP
 
 namespace ftxui {
-
-class Terminal {
- public:
-  struct Dimensions {
-    int dimx;
-    int dimy;
-  };
-  static Dimensions Size();
-
-  enum Color {
-    Palette1,
-    Palette16,
-    Palette256,
-    TrueColor,
-  };
-  static Color ColorSupport();
+struct Dimensions {
+  int dimx;
+  int dimy;
 };
 
+namespace Terminal {
+Dimensions Size();
+
+enum Color {
+  Palette1,
+  Palette16,
+  Palette256,
+  TrueColor,
+};
+Color ColorSupport();
+}  // namespace Terminal
+
 }  // namespace ftxui
 
 #endif /* end of include guard: FTXUI_CORE_TERMINAL_HPP */