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.

It used to be the case that frame advance skipped duplicate frames
(i.e. it would take 30 frame advances to get through one second
of emulated time in a 30 fps game), but this broke in 9c5c3c0.
Skipping duplicate frames making TASing less annoying.

Prior to this change, it's possible for m_wake_me_up_again to be used
while it's in an uninitialized state from the exposed API.

e.g.

- Using SetEnable after construction would perform an uninitialized read.
- Using PushEvent would perform an uninitialized read by way of operator |=.

internally, an uninitialized read can happen if PullEventsInternal() is
executed before other functions.

Just to avoid the whole possibility of performing uninitialized reads,
we just give the class member a default value of false.

BreakBeforeBraces: Allman apparently includes all styles,
except for AfterUnion (which is false) when using clang-format -dump-config

Replaces old and simple usages of std::atomic<bool> with Common::Flag
(which was introduced after the initial usage), so it's clear that
the variable is a flag and because Common::Flag is well tested.

This also replaces the ready logic in WiimoteReal with Common::Event
since it was basically just unnecessarily reimplementing Common::Event.

Also remedies places where the video backends and core rely on things
being indirectly included.