feat(graphic): add draggable support to group elements.

[pissang]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

The code change is mainly in the PR ecomfe/zrender#555

This PR is about adding a case to test draggle on the group elements.

Related issue

#5569

Usage

{
  type: 'group',
  draggable: true,
  children: [...]
}

[inkn]

when it can be used?

[pissang]

@inkn It has been included in the version 4.7.0 published a few days ago

[llcat]

@pissang hi, can i use the draggable setting in custom series renderItem function?, i have updated echarts version to 4.7.0, it seems no use, this is mine code:

return {
            type: 'group',
            position: [10, c2[1]],
            draggable: true,
            name: 'test',
            ondrag: function() {
                console.log('----drag')
            },
            children: [
                {
                    type: 'rect',
                    draggable: true,
                    ondrag: function() {
                        console.log('----drag')
                    },
                    shape: {
                        x: 10,
                        y: 10,
                        width: 20,
                        height: 20
                    },
                    style: {
                        fill: '#ccc'
                    }
                }
            ]
        }