New nebular components

[denStrigo]

Please read and mark the following check list before creating a pull request (check one with "x"):

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

[Tibing]

1. I don't think we need such a header on each new page:
   
   We haven't got this header before. And I think the user will always know where is he because he becomes on this page by clicking on the menu item with the appropriate title.
   So, If you think these headers required, please, explain why.

[Tibing]

2. I'm pretty sure we can position these showcases much better:)) so much free space now
   

[Tibing]

3. On the dialog page
   
   Let's add more examples and let the user see the difference between enabled and disabled functionalities. I mean let's create cards with two buttons, for example: one for dialog with component another for dialog with template. One for dialog with backdrop, one without.
   What do you think?
   For now this page looks slightly messy, because we have only one button for each card.

[Tibing]

4. The same thing for windows page
   
   This page very similar to the existing one, I mean, modals, so, let's build it the same way.

[Tibing]

5. May we add one more example on the chat ui page? So much space around
   

[Tibing]

6. so much space for each input, don't you think?
   

[Tibing]

7. Also, I'm pretty sure we have to remove previous versions of showcases from the app. I mean modals in UI features and form inputs in forms