#176 Add Platform theme

[aqwert]

PlatformTheme allows for under simple use cases a basic common theme with color. If any substantial differences between the material and cupertino themes are needed then setting the materialTheme and cupertinoTheme properties will be better

Adding documentation update in another commit

[fotiDim]

Could we have light and dark theme support in PlatformTheme? Right now I have resorted to:

PlatformApp(
...
             cupertino: (_, __) => CupertinoAppData(
              theme: MaterialBasedCupertinoThemeData(
                  materialTheme:
                      isDark ? materialDarkTheme : materialLightTheme),
            ),
...
)

which works but it would be nice if it was abstracted away in PlatformTheme.

[aqwert]

@fotiDim I am still working on how light and dark theme will work. CupertinoApp does not have a dark mode property, instead it uses CupertinoDynamicColor