feat: display 6 actions as contentless node

[yeze322]

Description

Follows new design, implement a new widget <ActionHeader /> to draw contentless node and apply it to 6 types:

1. EndDialog
2. EndTurn
3. RepeatDialog
4. CancelAllDialogs
5. LogAction
6. TraceActivity

Preview the changes in UISchema (2 types out of 6):

Design

According to new design, we want to show some types as contentless node.

What's changed?

1. Created a new widget <ActionHeader />
2. Consumed the new widget in 6 types' definition to make them borderless.
3. Adjusted element size constants.
4. Color

Why created a new widget?

I plan to retire the FormCard component in next few PRs because it can't meet our customization requirements under the new widget-based structure.
Therefore, this PR copied out a part of its code as ActionHeader; the new widget will be applied to more actions as a nested field. For example:

[SDKTypes.SendActivity]: {
    "ui:widget": Card,
    "header": {
        "ui:widget": ActionHeader,
    },
     "body": {
        "ui:widget": LGActivity,
        "text": data => data.activity,
    }
}

Task Item

fixes #1701

Screenshots

[a-b-r-o-w-n]

Is it possible to use the colors from the @ui-fabric/fluent-theme package?

[yeze322]

@a-b-r-o-w-n Yes! I would like to do that. We need designer's help on mapping those colors to our theme, do we need an issue to track this?