[Vala] gross string literals

[Sam Liddicott <sam liddicott com>]

protected static const string xmldef="blah";

is implemented in the C .h file as:
#define PHONE_UI_GLADE_xmldef "blah"

I get uneasy because in some of my cases the string could be a few
thousand characters long.

I'd rather it was implemented directly as a string constant, with
"extern" in the header file.

Sam