Kingroon kp3s mods

Some upgrades to Kingroon KP3s.

## PSU mount at the back of the printer
Position of PSU is not increasing footprint of the printer as from the left space is required for the wires anyway. All original PSU venting holes are left open.
Switch plate has to be fixed with longer M3 screws to the outside of the new mount.
Supports were required to print this part.

## Handle with guiding channel for filament
Main purpose is of course mobility, but also it has guiding channel for filament to go more straight from the top and avoid sharp bend into hotend. This channel is open from the top so that filament could move out of it when printing at higher heights, where this position is no longer suitable.
To fix to the extrusion, 2 holes at the extrusion top needs to be threaded to M5 and 2 additional 20mm bolts required.
No supports required when printing upside-down.

Both parts were printed with fuzzy skin setting enabled in Cura.