Reorganize tab popup code a bit more cleanly.

Rather than trying to reverse-engineer what kind of tab/workspace
popup to create from within meta_display_begin_grab_op(), just create
the popup directly from do_choose_window()/handle_workspace_switch()
after completing the grab, since they already know which kind they
want.

Also add meta_screen_destroy_tab_popup()/_destroy_workspace_popup()
rather than having meta_display_end_grab_op() poke into MetaScreen's
internals itself.

http://bugzilla.gnome.org/show_bug.cgi?id=580917

src/core/keybindings.c

@@ -3074,6 +3074,10 @@ do_choose_window (MetaDisplay    *display,
       return;
     }
 
+  meta_screen_ensure_tab_popup (screen, type,
+                                show_popup ? META_TAB_SHOW_ICON :
+                                META_TAB_SHOW_INSTANTLY);
+
   meta_ui_tab_popup_select (screen->tab_popup,
                             (MetaTabEntryKey) initial_selection->xwindow);
 
@@ -3455,6 +3459,7 @@ handle_workspace_switch  (MetaDisplay    *display,
 
   if (grabbed_before_release && !meta_prefs_get_no_tab_popup ())
     {
+      meta_screen_ensure_workspace_popup (screen);
       meta_ui_tab_popup_select (screen->tab_popup, (MetaTabEntryKey) next);
 
       /* only after selecting proper space */