Fix mark-message translation and keybind menu.

Fix mark-message translation and keybind menu.

Move the OP_MARK_MESSAGE and hotkey macro from MENU_GENERIC to the
MENU_MAIN keymap.  Putting the macro under generic prevents it from
overriding a keybinding in the index (even if the function is bound to
noop).  Additionally, the macro can only be executed from the index,
so it doesn't make sense as a generic keybinding.

Use the term "hotkey" in both the OPS and km_bind description.

Mark the km_bind description translatable.

Add L10N messages for the new translation strings.