Feature add rgbled getian gt p6 prgb4303

Adds 6-pin RGB LED  gt-p6-prgb4303.
Datasheet is available at https://www.gme.sk/img/cache/doc/518/177/vykonova-led-getian-gt-p6prgb4303-datasheet-1.pdf.
Silkscreen uses 0.15mm width, 0.15 mm clearence from pads and is visible after assembly. 
Silkscreen has a reference mark for pin 1, red anode.  Pin 1 is at top left.
Origin is at middle of part.
Pins are named by function "Red anode"->RA, "green cathode"->GC etc. to avoid ambiguity. 

Related library symbol: https://github.com/ojousima/kicad-library/commit/4468819620dfeaf375495d03069b5214a78cd73e.