qOverload is used to disambiguate pointers to overloaded functions, but
most of the usages in the code base were with non-overloaded functions.

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.

Provides the same behvaior, but in a much more concise manner.

Its usage was inconsistent, confusing, and buggy, so I opted to just
remove it entirely. It has been replaced with PadIndex for the
appropriate instances (mainly networking), and inappropriate usages
(where it was really just a player ID) have been replaced with the
PlayerId type. The definition of "no mapping" has been changed from -1
to 0 to match the defintion of "no player", as -1 (255 unsigned) is
actually a valid player ID.

The bugs never manifested because it only occurs with a full lobby of
255 players, at which point the last player's ID collides with the "no
mapping" definition and some undefined behavior occurs. Nevertheless, I
thought it best to fix it anyways as the usage of PadMapping was
confusing.

Basically everything here was race conditions in Qt callbacks, so I changed the client/server instances to std::shared_ptr and added null checks. It checks that the object exists in the callback, and the shared_ptr ensures it doesn't get destroyed until we're done with it.

MD5 check would also cause a segfault if you quit without cancelling it first, which was pretty silly.

Avoids dragging in netplay-related headers where they aren't explicitly
necessary.

Previously there was only one function under the NetPlay namespace,
which is kind of silly considering we have all of these other types
and functions existing outside of the namespace.

This moves the rest of them into the namespace.

This gets some general names, like Player, for example, out of the global namespace.