qz:steal support

[tresf]

Ability to modify the qz:launch behavior to gracefully shut down the OTHER instance of QZ Tray.

• Currently if QZ Tray is started, it will shut down the CURRENT instance of QZ Tray if another instance is running
• It may be beneficial to allow the new instance to "steal" the process and shut down the other
• This can have undefined results when multiple users are working simultaneously, it should only ever be deployed in environments where workstations are shared (not in a Terminal Services or Citrix style environment when they are using the workstations simultaneously)

The new feature should be usable through Command Line or HTML/URL:

• Command Line:

  java -jar qz-tray.jar --steal

• HTML/URL:

  <a href="qz:steal">Start QZ Tray, stealing it from another user if possible</a>

[tresf]

This is tested and working. Please check it out on the wiki: https://qz.io/wiki/multiple-instances