Create ARCodeWidget and GeckoCodeWidget once on startup rather than
every time a game is launched or shutdown.

In addition to losing focus on the tab (since the previous widget and
tab no longer existed), the behavior prior to this commit could cause a
crash if the user initiated a game shutdown and then opened a code edit
window since the AR/GeckoCodeWidget would get deleted in the meantime.

RetroAchievements does not allow cheats such as Action Replay or Gecko in hardcore mode, for fairness.

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.

Avoids propagating headers into scopes where they're not necessary.

Also uncovered reliance on an indirect inclusion within
CheatsManager.cpp, which is now fixed.