Additional "Window" menu bar element.

Element "Window" submenu should consist of checkable menu items in form of
"Show ..." (or other decisions about showing/hiding interface elements).

It contains:
* "Show average type" menu item which shows/hides average type
picker panel.
* "Show interface" menu item that will hide draw3 panes beside
draw graphs view. This is meant to allow for maximum clarity of
graphs on small screens/embedded devices.

+ Including polish translations.

Window checkboxes states are saved to configuration file ".draw3" in
form of bool (1/0) parameters of form "HIDE_<MenuItem>" (ex. "HIDE_AVERAGE").