[UX] Normalize SpinWidget and DoubleSpinWidget

- Use same names for options;
- Have them both movable
- Add option to keep widget shown on Apply (and use that
  when appropriate: when the value may have an immediate
  visual effect, so one can tweak it without having to
  go thru menus to re-open it again).

frontend/apps/reader/modules/readerrolling.lua

@@ -455,7 +455,7 @@ You can set how many lines are shown.]])
                     precision = "%d",
                     ok_text = _("Set"),
                     title_text =  _("Set overlapped lines"),
-                    text = overlap_lines_help_text,
+                    info_text = overlap_lines_help_text,
                     callback = function(spin)
                         G_reader_settings:saveSetting("copt_overlap_lines", spin.value)
                         touchmenu_instance:updateItems()