Keep "Waiting for Server Connection" panel on top of GUI when waiting for connection

@utkarsh.ayachit @cory.quammen @patchett2002

Connecting to a remote server can be very confusing for new ParaView users. This is a scenario that can cause a headache for the user and is embarrassing to run into when teaching the client-server use-case:

• Start a client-server (reverse connection). The panel with "Waiting for Server Connection" pops up.
• Something fails on the remote side. For example, the user input a non-existent Slurm partition, which causes the sbatch command to fail.
  • At LANL, we use xterm. When this happens, the xterm window echos an error, stays open for a short time, and closes. ParaView continues to wait for a server connection.
• User doesn't see the error message
• Clicking around the GUI happens to try and figure out what went wrong. Sometimes, the user is left looking at this:
  • It's not obvious that a grey renderview background means ParaView is still waiting for a connection.
• User tries to connect again and is given the error: createServer called while waiting for previous connection to be established.

The panel that lets you cancel the connection should stay on top of the ParaView GUI. I think this would be enough to prevent scenarios like this.