Use multiple library table files (lp:#1756735)

Original report created by Janis Skujenieks (janis-skujenieks)

This is wish for changes in functionality.

KICAD_SYMBOL_DIR and KISYSMOD environment variable would allow multiple
locations of libraries. Maybe allow semicolon separated directories. KiCAD
would search each location for sym-lib-table or fp-lib-table file and also
check current configuration directory. All appropriate files would be
joined together for final used list of libs.

This would make use of library repositories easy and wouldn't influence
current users. Currently, after I pull changes to my repo I need to fix lib
tables manually.

Original tags: eeschema feature.request pcbnew