Close button label text as a parameter

[lcuis]

This PR is proposing the possibility to specify the search dialog box Close button label text. Here is an example of use with the replacement of "Close" by "Cancel":

             new SearchableDropdown(
                items: items,
                value: selectedValue,
                isCaseSensitiveSearch: true,
                hint: new Text(
                  'Select One'
                ),
                searchHint: new Text(
                  'Select One',
                  style: new TextStyle(
                      fontSize: 20
                  ),
                ),
                onChanged: (value) {
                  setState(() {
                    selectedValue = value;
                  });
                },
                closeButtonText: "Cancel",//Replaces the "Close" of the search dialog box by a "Cancel"
              ),

I personally use this for translation.

[lcuis]

Closing this pull request because of new pull request with the clear button.