Support for only images as grid items is added

To support grid items as only images a new QuickActionImageGrid class is added. Here is a sample code how to use it:
```
public class QuickActionActivity extends SherlockActivity implements OnClickListener {

    private QuickActionWidget mBar;
    private QuickActionWidget mGrid;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Button btn = (Button) findViewById(R.id.btnShowImageGrid);

        btn.setOnClickListener(this);

        prepareQuickActionImageGrid();

    }

    @Override
    public void onClick(View view) {
        switch(view.getId()) {
            case R.id.btnShowImageGrid:
                onShowImageGrid(view);
                break;
        }
    }

    public void onShowImageGrid(View v) {
        mGrid.show(v);
    }

    private void prepareQuickActionImageGrid() {
        mGrid = new QuickActionImageGrid(this);
        mGrid.addQuickAction(new MyQuickAction(this, R.drawable.gd_action_bar_compose));
        mGrid.addQuickAction(new MyQuickAction(this, R.drawable.gd_action_bar_export));
        mGrid.addQuickAction(new MyQuickAction(this, R.drawable.gd_action_bar_share));
        mGrid.addQuickAction(new MyQuickAction(this, R.drawable.gd_action_bar_search));
        mGrid.addQuickAction(new MyQuickAction(this, R.drawable.gd_action_bar_edit));
        mGrid.addQuickAction(new MyQuickAction(this, R.drawable.gd_action_bar_locate));

        mGrid.setOnQuickActionClickListener(mActionListener);
    }

    private OnQuickActionClickListener mActionListener = new OnQuickActionClickListener() {
        public void onQuickActionClicked(QuickActionWidget widget, int position) {
            Toast.makeText(QuickActionActivity.this, "Item " + position + " clicked", Toast.LENGTH_SHORT).show();
        }
    };

    private static class MyQuickAction extends QuickAction {

        private static final ColorFilter BLACK_CF = new LightingColorFilter(Color.BLACK, Color.BLACK);

        public MyQuickAction(Context ctx, int drawableId) {
            super(ctx, buildDrawable(ctx, drawableId));
        }

        private static Drawable buildDrawable(Context ctx, int drawableId) {
            Drawable d = ctx.getResources().getDrawable(drawableId);
            d.setColorFilter(BLACK_CF);
            return d;
        }

    }
}
```