[Menu] NbMenu - Deselect all menu items when this.findItemByUrl() returns undefined

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
When there are two NbMenu components active on a single page (for example, in the ngx-admin demo) and they both contain different NbMenuItems, both NbMenus will retain the .active class on the last NbMenuItem that was clicked, even if the router path does not match either of the active links.

**Expected behavior:**
I see that the NbMenu component subscribes to router event changes, during which a pathMatch full or partial check is initiated by the selectFromUrl method. However, the selectUrlFrom method will only set the selected property to true if there is a match. In the case when there is no match, it should de-select any active links.

**Steps to reproduce:**
1) Create two NbMenu components on a page, each being passed different NbMenuItem arrays.
2) Click on a link from the first NbMenu component, verify that the router has navigated to the link and that the link has the .active class.
3) Click on a link from the second NbMenu component, verify that the router has navigated to the link and both NbMenu components have .active classes on the links that were clicked.

**Related code:**
N/A

### Other information:

**npm, node, OS, Browser**
```
Node: 10.13.0
OS: macOS High Sierra
Browser: Chrome 72.0
```

**Angular, Nebular**
```
Angular: 6.0
Nebular: 2.0.2
```