[Settings] Silence the analyzer about a (not) possible null pointer dereference.
[Settings] Initialize members in constructors.
[Settings] Remove unused variables.
[Settings] Operator=' should return &.
[Settings] Function parameter '...' should be passed by reference.
[Settings] Prefer prefix ++/-- operators for non-primitive types.
[Settings] Possible inefficient checking for 'settingMap' emptiness.
[Settings] Make functions static if possible.
[GUIControlSettings] Make GetID() const.
[AdvancedSettings] Removed unused member variable and moved m_initialized = true at the end of Initialize().
Merge pull request #2688 from ace20022/cleanup_settings

[Settings] Clean up things found by Cppcheck