When font loading is finished, restore the font list in the
corresponding async completion callback, and not in the
"notify::empty" signal handler of the fonts async set

This solves a problem arising when gimp_fonts_wait() is called
*inside* a "notify::empty" signal handler, emitted when reloading
fonts (causing the "empty" property of the fonts async set to
switch from TRUE to FALSE):  When the wait is over, "empty" will
switch back from FALSE to TRUE, however, since the "notify" signal
is non-recursive, the corresponding handler will not be called,
gimp_fonts_wait() will return *before* the font list is restored,
and the caller will see an empty font list.  This can happen under
certain circumstances when reloading fonts while the text tool is
active.