TreeTable | selectionKeys support for checkbox selection mode

[cetincakiroglu]

• New input property selectionKeys will be added to improve selection logic in checkbox selection mode.

Usage:
Users must use dataKey property along with the selectionKeys to build a context between the selection and data of the component. Corresponding dataKey value of the element can be placed inside the data object or outside.

// template
<p-treeTable [(selectionKeys)]="selectionKeys" dataKey="key">
    ....
</p-treeTable>

// ts
selectionKeys = {'0': {checked: false, partialChecked: true}, '0-1': {checked: true, partialChecked: false}}

example data:

{
    key: '0',
    data: {
        name: 'Applications',
        size: '100kb',
        type: 'Folder'
    },
    children: [
     ....
        {
            key: '0-1',
            data: {
                name: 'editor.app',
                size: '25kb',
                type: 'Application'
            }
        }
    ]
}

selectionKeys is not compatible with legacy selection property, users should choose one of them
selection property will be deprecated in the future and all selection logic will handle by selectionKeys

For the performance concerns children elements will not automatically selected on the initial load if parent's key provided in selectionKeys. All keys of the elements should be provided in selectionKeys.