Improved Preassembled Iris Box

This is a modification of Emmett's excellent Preassembled Iris Box. I've made the following changes:

* Extend the internal tabs to prevent the outer shell from lifting up when the box is closed. This lets you pick up the box when closed without worrying that the leaves will fall out of the top pegs.
* Add support at the interface between the base and the outer shell. This makes printing easier as the outer shell isn't printed on air anymore, but this has to be cut in post processing (see photos)
* Remove the sharp base and replace it with a much more rounded one that's easier on the hands.
* Increase the thickness of the bottom of the box to make it sturdier.

The HQ STL file was generated with the sphere resolution set to 160, the other STL is at 120.