Added support for JSON PlainTextTransferable to charts and subscriptions

Added support for JSON PlainTextTransferable to charts and subscriptions

With these changes dragging JSON text using
org.simantics.ui.dnd.PlainTextTransfer on top of model browser charts
and subscriptions and also on a chart editor is now supported.

The dragged JSON data must be in the following format:
<pre>
{
    "type" : "variable",
    "uri" : "variable space URI to dragged entity",
    "defaultPropertyUri" : "variable space URI to default referenced
property variable of the dragged entity"
}
</pre>

Only "type" and "defaultPropertyUri" keys are required.

refs #6844

Change-Id: I3684a879a6c80bbff812166d14aa28eac5244663