Text inside a form template inside a window is lost on window minimize

#1473 # Issue type

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

* [X ] bug report
* [] feature request

### Issue description

**Current behavior:**
In a NbWindow which has a element, I type in some text. On minimizing the window and un-minimizing it, the <textarea> is still there but the values I typed in are missing. The same is not the case with the full screen. That seems to keep the values intact.

**Expected behavior:**
After minimize/un-minimize/resize operations, the content should be retained.

**Steps to reproduce:**

1. Go to -> https://akveo.github.io/nebular/docs/components/window/examples#nbwindowservice
2. In the first example called "Showcase", click "open window"
3. It'll open a window with two fields "subject" and "text"
4. Type in something in one or both fields
5. Minimize the window and then un-minimize it or maximize it
6. The values you typed in will no longer be there.

**Related code:**
```
Not necessary as this behavior can already be seen in the official nebular website examples -> https://akveo.github.io/nebular/docs/components/window/examples#nbwindowservice
```

### Other information:

**npm, node, OS, Browser**
```
Chrome: Version 81.0.4044.138 (Official Build) (64-bit)
```

**Angular, Nebular**
```
Nebular-v5.0.0
```