Enable "Match Selected" only if there's a field selected *and* we can do

Enable "Match Selected" only if there's a field selected *and* we can do
a "Match Selected" on it - we can't do a "Match Selected" if the field
has no value (e.g., FT_NULL) and has a length of 0.

If we unselect the current packet, we don't have a protocol tree, so we
don't have a currently selected field - clear the "Match Selected" menu
item and the display in the status line of information about the
currently selected field.

Move the low-level statusbar manipulation into "gtk/main.c", in routines
whose API doesn't expose anything GTK+-ish.

"close_cap_file()" calls one of those routines to clear out the status
bar, so it doesn't need to take a pointer to the statusbar widget as an
argument.

"clear_tree_and_hex_views()" is purely a display-manipulating routine;
move it to "gtk/proto_draw.c".

Extract from "tree_view_unselect_row_cb()" an "unselect_field()" routine
to do all the work that needs to be done if the currently selected
protocol tree row is unselected, and call it if the currently selected
packet list row is unselected (if it's unselected, there *is* no
protocol tree, so no row can be selected), as well as from
"tree_view_unselect_row_cb()".

Before pushing a new field-description message onto the statusbar, pop
the old one off.

Get rid of an unused variable (set, but not used).

svn path=/trunk/; revision=3513