Before the call to OnSelectionChange, m_code_edit and m_code_remove are
disabled and UpdateList calls m_code_list->clear(), thereby deselecting
any selected items.

When no items are selected, OnSelectionChange disables m_code_edit and
m_code_remove and then returns. Since that was already done, the call
doesn't change anything and can be removed.

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.

Also some minor refactoring of nearby/related code:
* Make non-obvious variable types explicit instead of auto.
* Throw some consts around.
* Use setDisabled(empty) instead of setEnabled(!empty).

Move some #includes around to match the Contributing guidelines.

Move CheatManager's child widgets into scroll areas to allow making the
window smaller than the default.

In CheatSearchWidget, enable word wrapping for the label describing the
address space and search type to help it fit better inside a narrower
window.

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