Adds View->Voxel Edges to toggle voxel edge display, closes #21.
Fix: Correct save ambient occlusion display setting.
Fix: Fill tool could cause crash, closes #23.

Trying to fill voxels with a colour when they already were that colour would cause a crash.
Can hold down mouse button and drag with paint tool, closes #22
Changes to the DrawingTool, where it can now draw lines when the left mouse button is dragged
Solving issue #26, now with the possibility of adding multiple grids to the scene.
Fixed issue #29 - Grid resizing
Grid Manager is finally done. This solves parte of Issue #27. The other part involves making the grids collide with the newly created grids.
Merge branch 'master' of https://github.com/brunocanella/zoxel into develop