The inequality operator is automatically generated by the compiler if `operator==` is defined.

This way you can't mix up regular config callback IDs and CPU thread
config callback IDs. (It would be rather bad if you did!)

In theory, our config system supports calling Set from any thread. But
because we have config callbacks that call RunAsCPUThread, it's a lot
more restricted in practice. Calling Set from any thread other than the
host thread or the CPU thread is formally thread unsafe, and calling Set
on the host thread while the CPU thread is showing a panic alert causes
a deadlock. This is especially a problem because 04072f0 made the
"Ignore for this session" button in panic alerts call Set.

Because so many of our config callbacks want their code to run on the
CPU thread, I thought it would make sense to have a centralized way to
move execution to the CPU thread for config callbacks. To solve the
deadlock problem, this new way is non-blocking. This means that threads
other than the CPU thread might continue executing before the CPU thread
is informed of the new config, but I don't think there's any problem
with that.

Intends to fix https://bugs.dolphin-emu.org/issues/13108.

Because of the previous commit, this is needed to stop DolphinQt from
forgetting that the user pressed ignore whenever any part of the config
is changed.

This commit also changes the behavior a bit on DolphinQt: "Ignore for
this session" now applies to the current emulation session instead of
the current Dolphin launch. This matches how it already worked on
Android, and is in my opinion better because it means the user won't
lose out on important panic alerts in a game becase they played another
game first that had repeated panic alerts that they wanted to ignore.

For Android, this commit isn't necessary, but it makes the code cleaner.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

DualShock UDP Client is the only place in the code that assumed OnConfigChanged()
is called at least once on startup or it won't load up the setting, so I took care of that

The goal of this change is to make Config::Get(const Info<T>&)
fast so that we can use it in hot paths.

The way Config::Get works in master, it first calls
Config::GetActiveLayerForConfig which searches for the
setting in all layers, and then calls Config::Layer::Get
which searches for the same setting again within the given
layer. We can remove this second search by combining the
logic of Config::GetActiveLayerForConfig and
Config::Layer::Get into one function.