Add Icon to NbSelect (Clear selection)

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
I would like to clear an individual nb-select and would like to give the user a X icon on the select bar for it. Currently, there is no support to add or enable another icon other than the drop-down-chevron on the nb-select.

**Expected behavior:**
Add provision to add or show a clear icon button on the select drop down.

**Steps to reproduce:**
Add an to the project.

**Related code:**
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Office A

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A sample application via GitHub

StackBlitz [StackBlitz Seed Project](https://stackblitz.com/github/akveo/nebular-seed)

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### Other information:

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