Make nb-layout optional

### Issue type

**I'm submitting a ...**

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
As I understand it I can't use components that require an overlay without also using `nb-layout`

**Expected behavior:**
I should be able to use components without `nb-layout`.

I've like to use Nebular just for a few select components and so do not want to have to use `nb-layout`. Angular Material does not require a parent component like `nb-layout` for overlays to work so why does Nebular?

I appreciate that I may not have the full story here, and am really only after some clarity as to whether my assumptions are correct and whether or not you plan to change it.