change where marks (as shown in empire menu view) are calculated

They are now calculated at the same time as other stats such as
ship power. There shouldn't be much increase in processor load, as
iterations over all systems were already done at the start of each
turn (from menu choose view).
The marks of each major in each category are now saved in statistics class,
and can probably be used to decide which one should be declared war
and such (instead of just shippower which is stupid).
cppcheck: remove unused variable
a little debugging friendlyness
implement weakest major ai joining the strongest