BWindow: use Unicode lowercase method
Update semantic shortcuts

Update BKeymap::GetModifiedCharacters() to translate a given character
and set of modifiers filling out a list of all characters that match for another
set of modifiers.

This allows us to, for example, get all characters in the normal map that
have the '+' character in the corresponding shift map.

It is fully generic allowing one to get a list of characters in any map given
a character and modifiers of another map.

Also I converted from using a BList to using a BObjectList.

With this, along with BWindow::HasShortcut(), the semantic shortcuts now
work not only with Command+'=', but any key in the normal map that has
'+' in it's shift map as long as it isn't already taken by another shortcut.