The memory usage slider now actually works.

+ Fixing type of Options::memUsage() to double

+ Changing default value of slider to 8

+ Making sure that settings apply correctly in slotApply()

+ Using Options::memUsage() in the maglim formula in StarComponent. I
cooked up the formula ad-hoc, but it seems to work pretty well. The
typical memory usage at small zoom levels with the slider at value
'2' is about 50K (total) and is about 90K with the slider at value
'10'. I found that 8 gives a good star density, to my satisfaction.