send_kbd_event: Fix accesskeys when priority = 0

With document.browse.accesskey.priority = 0, accesskeys were ignored if
the current tab was not the last (rightmost) tab.  This commit fixes the
problem.

Add a function tabs_are_on_top(struct terminal *) which returns
a Boolean value indicating whether the topmost window is a tab (as
opposed to, e.g., a menu).

This commit fixes commit 1bd498cde9.

src/terminal/tab.c

@@ -61,6 +61,16 @@ found_pos:
 	return win;
 }
 
+/** If the topmost window is a tab, return 1; else, return 0. */
+NONSTATIC_INLINE int
+tabs_are_on_top(struct terminal *term)
+{
+	if (list_empty(term->windows))
+		return 0;
+
+	return ((struct window *) term->windows.next)->type == WINDOW_TAB;
+}
+
 /** Number of tabs at the terminal (in term->windows) */
 NONSTATIC_INLINE int
 number_of_tabs(struct terminal *term)

src/terminal/tab.h

@@ -10,6 +10,7 @@ struct uri;
 struct window *
 init_tab(struct terminal *term, void *data, window_handler_T handler);
 
+int tabs_are_on_top(struct terminal *);
 int number_of_tabs(struct terminal *);
 int get_tab_number(struct window *);
 int get_tab_number_by_xpos(struct terminal *term, int xpos);

src/viewer/text/view.c

@@ -1525,8 +1525,11 @@ quit:
 			deselect_mainmenu(win->term, win->data);
 			print_screen_status(ses);
 		}
-		if (ses->tab != ses->tab->term->windows.next)
+		if (!tabs_are_on_top(ses->tab->term)) {
+			/* The event opened a menu; we're done. */
 			return NULL;
+		}
+		/* Otherwise, the event still needs to be handled. */
 		get_kbd_modifier(ev) |= KBD_MOD_ALT;
 
 		if (doc_view