SidebarService - toggle doesn't trigger onCollapse and onExpand.

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**

as per title
**Expected behavior:**

if toggle changes the state to collapse - on collapse should be called, otherwise - onexpand
**Steps to reproduce:**

**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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