Deduplicates callback definitions and situates it in one place.

This formats all copyright comments according to SPDX formatting guidelines.
Additionally, this resolves the remaining GPLv2 only licensed files by relicensing them to GPLv2.0-or-later.

This completes the removal of the old UUID implementation.

The default constructor of UUID doesn't initialize its data members, so
we need to directly initialize it to be invalid.

Creates a new BasicSettings class in common/settings, and forces setting
a default and label for each setting that uses it in common/settings.
Moves defaults and labels from both frontends into common settings.
Creates a helper function in each frontend to facillitate reading the
settings now with the new default and label properties.

Settings::Setting is also now a subclass of Settings::BasicSetting. Also
adds documentation for both Setting and BasicSetting.

- Removes a dependency on core and input_common from common.