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.

[REUSE] is a specification that aims at making file copyright
information consistent, so that it can be both human and machine
readable. It basically requires that all files have a header containing
copyright and licensing information. When this isn't possible, like
when dealing with binary assets, generated files or embedded third-party
dependencies, it is permitted to insert copyright information in the
`.reuse/dep5` file.

Oh, and it also requires that all the licenses used in the project are
present in the `LICENSES` folder, that's why the diff is so huge.
This can be done automatically with `reuse download --all`.

The `reuse` tool also contains a handy subcommand that analyzes the
project and tells whether or not the project is (still) compliant,
`reuse lint`.

Following REUSE has a few advantages over the current approach:

- Copyright information is easy to access for users / downstream
- Files like `dist/license.md` do not need to exist anymore, as
  `.reuse/dep5` is used instead
- `reuse lint` makes it easy to ensure that copyright information of
  files like binary assets / images is always accurate and up to date

To add copyright information of files that didn't have it I looked up
who committed what and when, for each file. As yuzu contributors do not
have to sign a CLA or similar I couldn't assume that copyright ownership
was of the "yuzu Emulator Project", so I used the name and/or email of
the commit author instead.

[REUSE]: https://reuse.software

Follow-up to 01cf05bc75b1e47beb08937439f3ed9339e7b254

The behavior of the Hotkey constructor is already accomplished via in-class member
initializers, so the constructor is superfluous here.

* Adds a new Hotkeys tab in the Controls group.
* Double-click a Hotkey to rebind it.

Instead, we make a proper registry class and house it within the main
window, then pass it to whatever needs access to the loaded hotkeys.

This way, we avoid a global variable, and don't need to initialize a
std::map instance before the program can do anything.