Rather than creating a local "global" everywhere we need it, just create
a single global "global".

This patch creates the global from main.js:start(), by doing

    window.global = Shell.Global.get();

where "window" means the global JS namespace, just like in a web browser,
because gjs has chosen to be wacky in that way. (If you remove "window.",
it still works but gjs warns about the assignment to an undeclared
variable. If you try to declare it with "let" or "var", it ends up being
scoped to Main rather than to the global object. So "window.global" seemed
best.

Alternatively, we could assign it from gnome-shell-plugin.c perhaps.