Use gint16 for GtkBorder

Use gint16 for GtkBorder

32K of border ought to be enough for any pixel dimensions. At least
until screens are so huge we start using doubles.

This saves a nice 64 bits of space when we have a GtkBorder
stored somewhere.

Signed integers are used to avoid surprising unsigned math
issues. Just search GTK's whole git log from inception
for "unsigned" if you want to find any number of commits
fixing signed/unsigned bugs.

https://bugzilla.gnome.org/show_bug.cgi?id=629387