[WIP] Draggable Dialog

还没有人认领这个 Issue。

As I know this is not part of Material Design spec.
But if you like to implement it in web, you have easy possibilities to do this with either [React](https://material-ui.com/demos/dialogs/#draggable-dialog) or [Angular](https://stackoverflow.com/a/49706221/3924697).

For phones I wouldn't like that feature, as you don't have any space to drag the dialog around, but for desktop applications or even on tablets this feature with a wider screens, makes sense, if you maybe like to see what was behind the dialog and it is somehow also Desktop-Application default/expected behavior for dialogs.

## Current implementations
There are two styles, which can be applied to the dialog host.
1. (default) - this style is based on a `Popup` (`PopupEx`) for the dialog.
2. `MaterialDesignEmbeddedDialogHost` - this style is based on `Grid` for the dialog.

## Current behavior
The current behavior is, ...
- that a `Card` is set as the `Grid` or `Popup` content.
- that the dialog will be shown centered to the dialog host.
- that the content (`DialogHost.DialogContent`, `DialogHost.DialogContentTemplate` etc.) is shown in the Card content.
- that while resizing/dragging the dialog remains centered.

## TBD
### default
- [ ] As implemented as a popup the drag could be done anywhere on the screen and is not limited to the dialog host. Should this be allowed or should it be limited?
- [ ] Where should the dialog move on resize or drag of the host window? I think `PopupEx` has already the right behavior, if the dialog is drawn in the window. But if you drag the popup out of the window boundaries should there still be a repositioning?
### MaterialDesignEmbeddedDialogHost
- [ ] The drag here could be realized with margins. In opposite to the default style, this is already bounded to the dialog host content or maybe window content.
- [ ] As working with margins, on resize the dialog could disappear. Here we need to develop a resize/maximize/restore strategy.
### both
- [ ] What should happen, if the dialog reopens? Should the last position persist (this would be kind of the default behavior, as the properties of the `Grid`/`Popup` would be modified on drag)?
- [ ] Should this feature be opt-in or on by default (this would be a breaking change though)?

首先查看当前的 Popup/PopupEx 对话框实现以及 MaterialDesignEmbeddedDialogHost 实现。在为两种样式实现并验证可拖动对话框之前，先解决所列出的有关边界、调整大小、重新打开、持久化和 opt-in 行为的决策。

由索引模型根据 Issue 内容生成。