snap together desktop catapult +

A small put reasonably powerful working catapult made of 4 parts that doesn't require any glue.

I've modified the excellent catapult by Julius3E8 as follows:
1 - the catapult arm is now constrained so it doesn't fall out the bottom of the base
2 - modified the firing lever so it now has a spring (end is constrained in the base)
3 - added a strengthen bar to stop the base flexing when firing
4 - reduced the gap between hook and lever so it is a snug fit
5 - modified the catapult arm so it lines up better with the catch when pulled back