Add black version of system tags icon

Includes PNG (16x16px) and SVG (scoured) versions in black, to be uniform with the other icon files.

apps/systemtags/img/tag.svg

 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <svg xmlns="http://www.w3.org/2000/svg" height="16" width="16" version="1.0">
  <rect style="color:#000000" fill-opacity="0" height="97.986" width="163.31" y="-32.993" x="-62.897"/>
- <path d="m6.1861 0.96121-4.807 0.01316c-0.27161 0-0.34656 0.20987-0.34703 0.34604l-0.015132 4.8017 8.7232 8.7222c0.19688 0.19829 0.51234 0.19697 0.7092 0l4.4569-4.4569c0.19672-0.19688 0.19895-0.51327 0.002-0.71024l-0.0011 0.00876-8.3668-8.3668-0.3541-0.35417zm-2.1669 1.8522a1.1438 1.1438 0 0 1 1.144 1.144 1.1438 1.1438 0 0 1 -1.144 1.144 1.1438 1.1438 0 0 1 -1.144 -1.144 1.1438 1.1438 0 0 1 1.144 -1.144z" fill="#6e6e6e"/>
+ <path d="m6.1861 0.96121-4.807 0.01316c-0.27161 0-0.34656 0.20987-0.34703 0.34604l-0.015132 4.8017 8.7232 8.7222c0.19688 0.19829 0.51234 0.19697 0.7092 0l4.4569-4.4569c0.19672-0.19688 0.19895-0.51327 0.002-0.71024l-0.0011 0.00876-8.3668-8.3668-0.3541-0.35417zm-2.1669 1.8522a1.1438 1.1438 0 0 1 1.144 1.144 1.1438 1.1438 0 0 1 -1.144 1.144 1.1438 1.1438 0 0 1 -1.144 -1.144 1.1438 1.1438 0 0 1 1.144 -1.144z"/>
 </svg>