[script] Find and interact with Clients
This patch adds mechanisms to find and interact with clients to the scripting API:
- workspace.clientAt provides a way to list the clients at a given location on the screen.
- workspace.clientListStackingOrder provides a list of all clients in the order they are stacked on the screen.
- workspace.raiseClient provides a mechanism to bring a given client to the top of that list, making it visible above all others.