MenuItem - Provide onClick action for a menu item

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description
I want open a `NbWindowComponent` when clicking on a menu item.

**Current behavior:**
There is no option to provide onClick action for a menu item.

**Expected behavior:**
I like to have onClick option for a menu item so that, i can do a action while clicking on a menu rather to navigate.

**Steps to reproduce:**

**Related code:**

```
insert short code snippets here
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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
```
Angular 8.0.1
Nebular 4.0.0-rc.7

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