headerbar: don't use GTK_PACK_* as indices into array

headerbar: don't use GTK_PACK_* as indices into array

The size allocation code maintains an array of two elements,
to track the allocation of children packed into the two sides
of a header bar. Sometimes this array is indexed with 0 and 1,
and sometimes its indexed with GTK_PACK_START and GTK_PACK_END.
The latter happen to have the values 0 and 1, respectively, but
that's not really obvious.

For clarity, this commit changes the code to index those arrays
consistently, sticking to 0 and 1 across the board.