Fix issues in RobotIKGui plugin

Issue #1 (closed):

I couldn't reproduce the issue, but it's pretty obvious that one of the proxies is null and causes the crash when one of the sensors tries to access them. Proxies are no longer set to null when disconnecting now.

Issue #2 (closed):

Manipulator now sets a boolean flag when it is moved. A new inverse kinematic is only computed if this flag is set.