[Popover] take into account position of parent elements

### Issue type

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

* [ ] bug report
* [ ] feature request

### Issue description

**Current behavior:**

Parent element position could be static, this should be taken into account.
**Expected behavior:**

**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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