Close unused mode setting and rendering devices

This achieves a few of things:

• It splits out device file management from MetaLauncher into a thread safe device pool object. It's possible to "open" the same file from multiple places, without actually opening it multiple time in parallel.
• It takes a step towards decouple mode setting devices from rendering by introducing using these "device files" directly in various situations; it doesn't go all the way, but it's a step in that direction
• It adds device file usage tracking, which allows for knowing when a device isn't used by anything. If a device isn't used by anything, it can be closed, which e.g. means drivers can be unloaded.
• On monitor reconfiguration, or on hotplug events, device files are reopened, and will stay open until they are no longer needed again.

Marked as draft as it includes !1822 (merged).