Add useful factory constructos to ThemeSwitcher

[EStepiuk]

Add useful switcher() and withTheme() factory constructors to ThemeSwitcher.

• switcher() adds switcher (ThemeSwitcherState) argument to builder:

  ThemeSwitcher.switcher(
    builder: (context, switcher) {
      ...
      onTap: () => switcher.changeTheme(
        theme: newTheme,
      );
      ...
    },
  );

• withTheme() also add current theme (ThemeDate):

  ThemeSwitcher.withTheme(
    builder: (context, switcher, theme) {
      ...
      onTap: () => switcher.changeTheme(
        theme: theme.brightness == Brightness.light
            ? darkTheme
            : lightTheme,
      );
      ...
    },
  );