[patch] timeline for headquarters

[Dwachs]

This enables timeline for headquarter buildings.

It does not require a new makeobj, however pak-set authors will have to change their dat-files to enter meaningfull intro/retire dates. The default in makeobj is 1900 and 2999, this means there will be no headquarters available before 1900 in pak64 and pak.german with this patch.

[The Hood]

Another good improvement

I will be using this in pak128.Britain.

[jamespetts]

Excellent!

[Dwachs]

Incorporated in 3563.

Pak-sets may need to set intro/retire dates for their headquarters, as the default values are 1900 and 2999, that is without proper intro dates no headquarters can be built before 1900.

Further requirement: to be able to upgrade a headquarter, a headquarter building for the next level has to be available.