Fix FPS stutter on rotating in place

- implements VoxelSystem::treeIsBusy() to allow tasks to not call blocking tree operation if the tree is busy
- change "Hover Voxel" code to not attempt to ray pick a voxel if the tree is currently busy. This is what actually fixes the stutter.
- breaks up Application::update() into smaller helper functions to help isolate problems that cause FPS stutter
- added some additional debugging for timing and on screen stats