Changes to the DrawingTool

Now, when the left mouse button is dragged in the DrawingTool, more voxels will be added.

Unfortunatelly, because of a limitation from the collision detection algorithm, (where it can only collide with the Y plane when there are no voxels around), there are some limitations for my implementation.

1. You can't add voxels in the same direction (x,y or z) than the one that was choosen in the on_activated method. For example, if the face you selected was Top (or None, because of the Y plane), then you will be able to insert new voxels in the X and Z planes.

2. You can only add new voxels when you hit the Y plane, or when you hit a face of a created voxel (Ex: if you drag the mouse over a LEFT, RIGHT, FRONT or BACK face of a Y created voxel). This is due to a limitation in the collision detection algorithm, where it can only collide with the Y plane.

I think I will be trying to modify the collision detection algorithm work with the X and Z planes next, so that I can remove the MUST PASS THE MOUSE OVER A FACE limitation, and calculate the new positions for the Voxels based on an offset of the first_voxel_selected's position;