NbSidebarComponent good to have ability to get state dynamically

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
Current implementation of `NbSidebarComponent` and `NbSidebarService` expect receiving state of sidebar via `getSidebarState` and `getSidebarResponsiveState` once. So to receive updated state consumer have to subscribe again.

**Expected behavior:**
Nice to have an opportunity of dynamically updated state in existing subscription.

**Steps to reproduce:**
Create a component that uses `NbSidebarComponent` and subscribe to `getSidebarState` observable.
Change sidebar state via for example `collapse()` method
Noticed no values received in subscription due to it is completed.