Bug 774906 - Replace File>New default ppi of 72...

... with something more suitable.
72 PPI is from a time where people thought this was a common screen
resolution. This is not the case nowadays, and anyway images targetted
for screen display should not bother with PPI resolution at all, only
with actual pixel dimensions.
PPI resolution is more useful for printing. And for this case, 300 is
quite an accepted OK value for most cases. So this is likely a better
default for GIMP.