Feature/49 add box shadow

[Ahmadre]

Related Issues

• boxShadow property for the selected item #49

Context

This PR adds both SalomonItemDecoration as a global Decoration for each selected and unselected Items and also add boxShadow overrides for each item, so that users can control globally as seperatly the boxShadow.

I decided to create a controlled DataClass SalomonItemDecoration instead of Flutter's Decoration so users can't misuse the supported Decorations.

Example:

Global:

bottomNavigationBar: SalomonBottomBar(
          itemDecoration: SalomonItemDecoration(
            selectedBoxShadow: [
              BoxShadow(
                color: Colors.blue.withOpacity(.5),
                blurRadius: 25,
              ),
            ],
          ),
          ...

In each item:

items: [
            /// Home
            SalomonBottomBarItem(
              icon: Icon(Icons.home),
              title: Text("Home"),
              selectedColor: Colors.purple,
              selectedBoxShadow: [
                BoxShadow(
                  color: Colors.red.withOpacity(1),
                  blurRadius: 20,
                ),
              ],
            ),
          ...

@lukepighetti @imaNNeo please review this if you got time.

[Ahmadre]

@lukepighetti