sharePartOfScreen is fixed for all users. resumePartOfScreenSharing added. connection.DetectRTC.screen.extensionid added.

`connection.resumePartOfScreenSharing()` added.

Now if you'll invoke
`connection.sharePartOfScreen(...)` and a new user will join you;
existing part of screen will be auto shared with him.

It means that
sharePartOfScreen will work with all new/old users.

-. renegotiation
scenarios that fails:
-. 1) if chrome starts video-only session and
firefox joins with only audio
-. 2) if chrome starts with audio-only
session and firefox joins with only video
-. 3) if chrome starts only
audio and firefox joins with audio+video
-. renegotiation scenarios that
works:
-. 1) if chrome starts audio+video and firefox joins with only
audio or audio+video
-. 2) if both browsers has similar streams

You can
set `connection.DetectRTC.screen.extensionid="your-chrome-extensionid"`
to make sure inline (newly) installed chrome extension is quickly used
for screen capturing instead of prompting user to reload page once to
use it.

It means that install the chrome extension and
RTCMultiConnection will auto use it. Don't ask your users to reload the
page:

```javascript
connection.DetectRTC.screen.extensionid =
'ajhifddimkapgcifgcodmmfdlknahffk';
```