This commit adds the default theme context, which replaces
the need to manually check the project and the default theme
all the time; simplifies related code.
It also adds framework for custom theme contexts, to be used
by the editor. Custom contexts can be attached to any node,
and not necessarily a GUI/Window node. Contexts do no break
theme inheritance and only define which global themes a node
uses as a fallback.
Contexts propagate NOTIFICATION_THEME_CHANGED when one of their
global themes changes. This ensures that global themes act just
like themes assigned to individual nodes and can be previewed
live in the editor.
As many open source projects have started doing it, we're removing the
current year from the copyright notice, so that we don't need to bump
it every year.
It seems like only the first year of publication is technically
relevant for copyright notices, and even that seems to be something
that many companies stopped listing altogether (in a version controlled
codebase, the commits are a much better source of date of publication
than a hardcoded copyright statement).
We also now list Godot Engine contributors first as we're collectively
the current maintainers of the project, and we clarify that the
"exclusive" copyright of the co-founders covers the timespan before
opensourcing (their further contributions are included as part of Godot
Engine contributors).
Also fixed "cf." Frenchism - it's meant as "refer to / see".
