Added pid symbols for dark theme

icons/pid_symbols/dark/arrow.svg

icons/pid_symbols/dark/burst_disk.svg

icons/pid_symbols/dark/check_valve.svg

icons/pid_symbols/dark/flexible_connection.svg

icons/pid_symbols/dark/manual_valve.svg

icons/pid_symbols/dark/motor_valve.svg

icons/pid_symbols/dark/pressure_gauge.svg

icons/pid_symbols/dark/pressure_regulator.svg

icons/pid_symbols/dark/pressure_transducer.svg

icons/pid_symbols/dark/quick_connector.svg

icons/pid_symbols/dark/relief_valve.svg

icons/pid_symbols/dark/three_way_valve.svg

icons/pid_symbols/dark/vessel.svg

src/ui/panes/pid_drawing_tool/symbols.rs