Enable properties, methods, events to be virtual based on flag

[chenss3]

Once #1898 gets merged to its target branch, I will change the target branch for this PR to my working branch: user/chenss3/udkauthoring.

This is apart of the bigger change to modify the projections. Enable properties, methods, events to be virtual if the abstract_class flag is on. This is an example of what a modified projection will look like.