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.
58126e479c