feat: added layer tree icons

assets/icons/plus.svg

@@ -29,7 +29,8 @@
      inkscape:zoom="11.046421"
      inkscape:cx="33.766593"
      inkscape:cy="34.852918"
-     inkscape:current-layer="layer1" />
+     inkscape:current-layer="layer1"
+     showgrid="false" />
   <defs
      id="defs1" />
   <g
@@ -39,19 +40,19 @@
     <rect
        style="fill:#969696;fill-opacity:1;stroke:none;stroke-width:5;stroke-linecap:round;paint-order:fill markers stroke"
        id="rect1"
-       width="3.4427652"
-       height="12.431063"
-       x="6.7452841"
-       y="2.2511349"
-       ry="0.49476871" />
+       width="1.8242012"
+       height="6.5867867"
+       x="7.5545654"
+       y="5.1732726"
+       ry="0.26216069" />
     <rect
        style="fill:#969696;fill-opacity:1;stroke:none;stroke-width:5;stroke-linecap:round;paint-order:fill markers stroke"
        id="rect2"
-       width="3.4427652"
-       height="12.431063"
-       x="6.7452841"
-       y="-14.682198"
-       ry="0.49476871"
+       width="1.8242012"
+       height="6.5867867"
+       x="7.5545659"
+       y="-11.760058"
+       ry="0.26216069"
        transform="rotate(90)" />
   </g>
 </svg>