* Always define GEANY_FOO_H to 1 in the header guards
* Always put a G_BEGIN_DECLS/G_END_DECLS guard in every header for
consistency, even private ones where it doesn't matter.
* Always include either <glib.h>, <gtk/gtk.h> or some other header
that will provide G_BEGIN_DECLS before using it. In a lot of headers
that use glib.h and gtk/gtk.h stuff anyway, this resolves an implicit
dependency they had on them being included before that header.
* Always put a comment at the #endif part of the guard so it's
easier to see what it applies to.
* Always use an underscore between the header guard identifier's words
even though the filename doesn't have one.