FR: Dynamic prevention / enabling of certain GSD components

[JoshStrobl]

By default, GNOME Settings Daemon runs all of its various components, even when they're not being used. While individually these components do not take a significant amount of total usage of the CPU (can vary from 300µs/s to 800µs/s), collectively they may result in a larger amount of events and wakeups, and negatively impact battery life.

Ideally, I'd like to see us detect when various components like external USB-based tablets are plugged in, and only start execution of the respective GSD components during that time (and proceed to stop execution / kill the process when the device is unplugged).