The m_verbosity_debug button was only conditionally being added as
widget, this was done in order to hide the object, but this left it
unmanaged.

Unconditionally adding it to the layout and controlling it's visibility
will resolve these issues

This replaces the MAX_LOGLEVEL define with a constexpr variable
in order to fix self-comparison warnings in the logging macros
when compiling with Clang. (Without this change, the log level check
in the logging macros is expanded into something like this:
`if (LINFO <= LINFO)`, which triggers a tautological compare warning.)

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Previously the logging was a in a little bit of a disarray. Some things
were in namespaces, and other things were not.

Given this code will feature a bit of restructuring during the
transition over to fmt, this is a good time to unify it under a single
namespace and also remove functions and types from the global namespace.

Now, all functions and types are under the Common::Log namespace. The
only outliers being, of course, the preprocessor macros.

Makes it easier for users to determine which option they need to
enable/disable as log messages only show the short name.

Also gets rid of two unnecessary casts.