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

Resolves trivial compiler warnings.

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.

This makes clang-format useful on those.

Also add a bunch of forgotten transitive includes, which otherwise
prevented compilation.