Added widget-stack that stacks widget like for tab group

[ralphocdol]

Addresses https://github.com/glanceapp/glance/issues/764

- size: small
  widgets:
    - type: group
      widgets:
        - type: stack
          title: Stack 1
          widgets:
            - type: hacker-news
              limit: 5
              collapse-after: 3
              cache: 5h
            - type: lobsters
              sort-by: hot
              tags:
                - go
                - security
                - linux
              limit: 5
              collapse-after: 3
              cache: 5h
                
        - type: stack
          title: Stack 2
          widgets:
            - type: custom-api
              title: C
              template: | #html
                <div>c</div>
            - type: custom-api
              title: D
              template: | #html
                <div>d</div>