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.
Reduces the amount of dependencies dragged in by the main window's
header. This also removes MainWindow.h includes elsewhere where they
aren't necessary, reducing the amount of UI files that need to be
recompiled if the main window's header changes.
