Remove window hints clearing the flag Qt::WindowContextHelpButtonHint,
which is already off by default in Qt 6.

In Qt 5 this flag was set by default for QDialogs, and on Windows put a
? button in the corner of the title bar allowing users to activate Qt's
QWhatsThis help system for a given widget. Since we don't set that text
the ? button was useless and so we hid it manually.

Fixes dynamically changing dpi scaling.
Load resources from svg if possible.
Currently svg support is not in Qt build in Externals,
and image files need to be added later.

Fixes a crash that could occur if the static constructor function for
the MainSettings.cpp TU happened to run before the variables in
Common/Version.cpp are initialised. (This is known as the static
initialisation order fiasco.)

By using wrapper functions, those variables are now guaranteed to be
constructed on first use.

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.