If we tried to write a switchable setting to config that was not using global in the global config instance, we could write the per-game setting accidentally. This ensures that we always use the global setting for global config and the currently applied setting for custom config.
Previously this could cause problems if a version of the template generated for WriteSetting didn't use the type you needed (e.g. floating point values). Now these are all ready without having to be used within frontend_common first.
SimpleIni only has the ability to use ANSI strings for config paths so this breaks opening configs on paths with special characters. This ensures that we open the right path on each platform.
Normally we save paths with '/' as the delimiter for each segment but when you manually select a network drive instead of using a mapped location, this would break. This ensures that if the read filesystem location starts with '//', we keep that pattern.
Replaces every way of handling config for each frontend with SimpleIni. frontend_common's Config class is at the center where it saves and loads all of the cross-platform settings and provides a set of pure virtual functions for platform specific settings.
As a result of making config handling platform specific, several parts had to be moved to each platform's own config class or to other parts. Default keys were put in platform specific config classes and translatable strings for Qt were moved to shared_translation. Default hotkeys, default_theme, window geometry, and qt metatypes were moved to uisettings. Additionally, to reduce dependence on Qt, QStrings were converted to std::strings where applicable.
