Use OS based Hotkey-Modifier FS#1958

Because ALT-GR fires a CTRL-ALT event on Windows systems, but ALT-GR is
needed to write square brackets on German Keyboards, CTRL-ALT can not be
used as modifier.

This patch introdues OS dependent modifiers. Mac systems continue to use
CTRL-ALT, all other systems use only ALT.

Let's hope this works out.