Option to snap to Y-Axis on brush tool.

[DylanBoxedCats]

In the editor view of the editor, I suggest the implementation of a Lock to y-axis checkbox for the brush tool. When enabled, the y-value for the cylinder/cuboid/pyramid/ cone will stay at the same y-level as it was when the mouse was first clicked. This would be useful for when using the fill tool to level out a certain area that already has hills when building custom terrain. As of now, when I go to use the brush tool, it always sets its Y-value to the lowest point of collision (drops it to the ground). This would be the behavior when the checkbox is unchecked.