Rename the monitor backend filenames

To match the "meta-*" new-style naming.
Move monitor manager headers to their own files

Instead of having them in monitor-private.h.

This way, it's easier to move our own backend when the chance arises.
Kill meta_ui_add_event_func / remove_event_func

The reason we don't simply use gdk_window_add_filter directly is
because of some twisted idea that any GDK symbol being used from
core/ is a layer violation. While we certainly want to keep any
serious GDK code out of ui/, event handling is quite important
to have in core/, so simply use a GDK event filter directly.
idle-monitor: Rearrange code to make it a cleaner split

We're going to push this to be subclasses soon.
idle-monitor: Split the DBus implementation to a separate file
idle-monitor: Move reset_idletime handling to events.c

It's not specific to the Wayland protocol at all, and it's not really
a Wayland compositor thing. This should eventually be in the native
backend.
idle-monitor: Create separate subtypes for backend-specific idle monitors

The separation here isn't 100% clean yet, as there's common
parts that are still protocol specific. We'll clean that up
in the next commit.
idle-monitor: Move watch implementations to be private to the subclass
idle-monitor: Move the alarms field to the XSync backend

For whatever reason, this hash table was in the generic
implementation section instead of the XSync implementation,
even though it's only used by the XSync implementation.

Use it as a first pass of things to move over.
idle-monitor: Kill off an unused field
idle-monitor: Make the rest of the implementation-specific fields private
cursor-tracker: Kill off an unused include