Resolves a warning where not all control paths return a value.

Migrates a remaining common file over to the Common namespace, making it
consistent with the rest of common files.

This also allows for high-traffic FS related code to alias the
filesystem function namespace as

namespace FS = Common::FS;

for more concise typing.

This behavior is already provided by the built-in exec() function. We
just need to check the return value of it.

Qt uses a signed value to represent indices. We should follow this
convention where applicable to avoid unnecessary sign-conversion
warnings, as well as making it easier to interoperate with other aspects
of Qt.

While we're at it, we can also make a sign-conversion explicit.

This is a user-facing string, so it should be marked as translatable.

Makes for shorter code, while also not requiring the buttons to be
directly translated, they'll be handled by Qt itself.

Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.

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

Presents profiles in a list, similar to switch.