Add client.enabled property.

[jheer]

• Add client enabled property. When enabled is set to false, the client will not initialize nor respond to query requests and selection updates. This setting can improve performance, for example by suppressing updates to off-screen interface components. When enabled is set to true, any pending initialization or query requests will then proceed.
• Add enabled option to makeClient helper utility.
• Refactor initialization logic between coordinator and client.
• Add tests for client enabled property.

[domoritz]

Awesome. I suppose this will help with #461.

I am a bit concerned about the name. An active selection/clause means it's the last interacted with but an active client means it's a client that is enabled. Maybe that's okay but as an alternative, we could call it enabled. I don't feel strongly about it, though.

[jheer]

I prefer enabled as well. I'll make that change, then merge!