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.