Fixing the experience problem

[Dávid]

In the original game, ships accumulated experience with each battle.
Unfortunately, this experience became useless, as the ships' crew was
not transferable to a new vehicle on upgrade. All new ships started at
0.
I think I have an idea to solve this: Experience pool. Each ship
gathers experience, which can be assigned or unassigned from ships,
similarly as fighters or other vessels. On the equipment screen, you
can freely assign/unassign XP from the ships:
Unassign XP from old ship -> Assign it to the new ship. This way, the
XP is preserved.
We could also say, in a way, this is not actually a bare XP transfer,
but rather the experienced crew gets transferred and replaced by
inexperienced crew...