Turn dialog used for hotseat mp games now applies a blindfold

This is intended to address bug #3856.

Following discussion on IRC, this is considered a feature request
and not a bug, and so cannot be addressed during feature freeze
for 1.12. Hence I am pushing it to a topic branch.

There is still a bug associated with my fix here, which is that unit
globes and health bars are still drawn through the blindfold, even
though they are not drawn for blindfolded observers. The cause of
this is that the get_visible function in unit.cpp will always
say that a unit is visible to the side controlling it, even if there
is shroud on the hex, which conflicts with what display.cpp says.

I can't push a changelog change either as we don't know what version
this will go in, but the changelog entry should be something like:

"Fix bug #3856: The turn dialog used in hotseat MP play now applies
a blindfold to the map for the duration of the dialog."