Invasion Transport Success needs a timer or abigger sinking

[tbr]

Currently an Invasion is a success (75000 in-scenario VP and therefore likely "troops ashore" after) if the prescribed number of transports REACH the point of disembarkation.

However, disembarking and amphibously assaulting takes time. If your invasion transports get sunk or significantly damaged within the firts 4-6 hours or so of the start of even a "small colonial" invasion your invasion is a failure, a very expensive failure. With 1.15 we get a notification of "enemy troops landing" and sucessfully reacting to this, e.g. by pushing through in an (even suicidal) attack on the transports with an inferior force and sinking the invasion trasnports should be rewarded as a victory.

There are basically two ways to achieve this:

1) Provide sufficient "in scenario" VP for sunk invasion transports so that sinking more than 2 thirds of them results in more than 75000 VP (would also have the benefit of simulating the extreme consequences of loosing e.g. an army/marine division)

2) Use a timer for the award of "invasion success" in-scenario VP. Currently, in long scnarios, the transports eventually leave the disembarkation point again, so there is already a timer. 6-10 hours would still be rather short but should work within the context of this game.