Unlike in DolphinQt, there isn't much space to show playtimes directly in
the game list, so I've put it in the game details dialog instead.

These files cannot compile correctly as C, so there's no reason to have
ifdefs for C compatibility.

We switched to not checking the __cplusplus macro in our JNI code some
time ago, but it seems like I forgot to remove it from these two files.

Yes, that's right! It's time to add even more NKit warnings,
because users still don't understand what NKit is or how it works!

More specifically, some users seem to be under the impression that
converting an NKit file to for instance RVZ using Dolphin's convert
feature will result in a normal RVZ file, when it in fact results in
an NKit RVZ file (since NKit is not a container format in the sense
that GCZ/WIA/RVZ/WBFS/CISO is, but rather a kind of trimmed ISO).
I can hardly blame users for not knowing this, because it's not
intuitive unless you know the technical details of how NKit works.

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.

Static methods use jclass, and non-static methods use jobject.

We generally have no reason to call these functions on our own, so
there's not much reason to declare them, especially not in the cpp
file where they're defined. In case we ever do get a reason to do
it, we can add declarations for just the functions that need them.

To make people more aware that they're not using a normal disc image.