Small Building Improvements Update

This update adds click-and-drag building, making it much faster and easier to place large numbers of blocks.

I've also made improvements to building performance in large worlds. While there are still some slowdowns when working on very large builds, performance is noticeably better, and I'm continuing to optimize it as development progresses.

As always, thank you for the feedback and bug reports. they've been incredibly helpful in improving BitVoxel.