LinkPreview widget

[Tishka17]

1. If there is now LinkPreview in window - it is disabled. Can be also disabled in widget itself
2. If it is rendered before ant text - show_above_text=True. Can be overriden by widget itself.
3. URL - is any Text widget
4. prefer_small_media and other bool flags can be set in widget

For example, in this code, link is shown above or below the text depedinging on when

Const("A", when="a")
LinkPreview()
Const("B", when="b")

[Tishka17]

this feature requires refactoring in window rendering logic