Make search widget toggable with menu item or toolbutton.

 * Add new signal "show_search" in UiSearch to synchronize Button and menu
   item.
 * Add menu item for gdl grip hide/locking.
 * Add accelerators for gdl grip hide/locking and search show/hide.
 * Add -DGDL_LESS_3_6 to decide between new is_closed() or old flags field.
 * Standardize menu naming (item_MENUPART_NAME).