Updated build menu

- Bottom build menu updated. Instead of the ugly buttons I added new images of the currently buildable objects in the expected final design. These will definitely change in the future, I simply made this change to give the players a glimpse of what my plan is with the UI when someone who knows how to do it properly will actually do it. :)

- During building objects, the box colliders are disabled on the mines so the placement indicator will still be visible when the mouse is over the mine. This was a really annoying bug so far, but it is now finally fixed. :)