Silence a few -Wsign-compare warnings.

AC_U: Added a stub for GetWifiStatus

PTM_U: Implemented the GetShellState function.

Fixed viewport error caused by rounding

Dsp service improvements

CFG:U: Implemented the GetCountryCodeID and GetCountryCodeString

Merge Config::ReadXYZs

Mutex: Changed behavior to always release mutex for all threads.

Svc and thread fixes

GLFW: Add an error callback before calling glfwInit()

dyncom: Use unordered_map rather than the terrible 2-level bb_map

arm_dyncom_interpreter: Get rid of unused var warnings

It will print a message to know what happened in case something went wrong in a GLFW call.

Also replace every printf() in the glfw emu-window by ERROR_LOG().

Seems (probably just placebo/wishful thinking) to make it slightly
faster. Also reduces memory usage and makes shutdown when debugging from
MSVC fast.

Fixed formatting and switch statement warnings

Remove unused includes to common/thread.h

Error codes

All service calls in the CTR OS return result codes indicating the
success or failure of the call. Previous to this commit, Citra's HLE
emulation of services and the kernel universally either ignored errors
or returned dummy -1 error codes.

This commit makes an initial effort to provide an infrastructure for
error reporting and propagation which can be use going forward to make
HLE calls accurately return errors as the original system. A few parts
of the code have been updated to use the new system where applicable.

One part of this effort is the definition of the `ResultCode` type,
which provides facilities for constructing and parsing error codes in
the structured format used by the CTR.

The `ResultVal` type builds on `ResultCode` by providing a container for
values returned by function that can report errors. It enforces that
correct error checking will be done on function returns by preventing
the use of the return value if the function returned an error code.

Currently this change is mostly internal since errors are still
suppressed on the ARM<->HLE border, as a temporary compatibility hack.
As functionality is implemented and tested this hack can be eventually
removed.

This prevents them from interfering with other constants defined in
different namespaces.

Added DeleteFile and DeleteDirectory functions to FS:USER and the archives.

Add comment style notes to CONTRIBUTING.md

Closes #215