Prusa MK3 MMU2 blade holder improvements (old version)

Edit: new and improved version here: https://www.thingiverse.com/thing:3268500



The blade holder was very badly designed. I added pegs to hold the blade, and peek holes to confirm the blade is correctly positioned.

I also removed some material from the "holder" part so it would have less chance of touching/grinding against the fixed part of the selector, and enlarged the filament feed coutout so misalignment is less of an issue.

Based on the GIT version of the parts.

Replaces "sel