Use a gray invisible icon for the tree

src/Gui/Icons/TreeItemInvisible.svg

@@ -2,20 +2,20 @@
 <!-- Created with Inkscape (http://www.inkscape.org/) -->
 
 <svg
-   xmlns:dc="http://purl.org/dc/elements/1.1/"
-   xmlns:cc="http://creativecommons.org/ns#"
-   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-   xmlns:svg="http://www.w3.org/2000/svg"
-   xmlns="http://www.w3.org/2000/svg"
-   xmlns:xlink="http://www.w3.org/1999/xlink"
-   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
-   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
    width="64px"
    height="64px"
    id="svg2816"
    version="1.1"
-   inkscape:version="0.91 r13725"
-   sodipodi:docname="TreeItemInvisible.svg">
+   inkscape:version="1.3.2 (091e20ef0f, 2023-11-25, custom)"
+   sodipodi:docname="TreeItemInvisible.svg"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:xlink="http://www.w3.org/1999/xlink"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:dc="http://purl.org/dc/elements/1.1/">
   <defs
      id="defs2818">
     <linearGradient
@@ -509,8 +509,8 @@
      inkscape:pageopacity="0.0"
      inkscape:pageshadow="2"
      inkscape:zoom="5.5"
-     inkscape:cx="35.267952"
-     inkscape:cy="19.835481"
+     inkscape:cx="35.363636"
+     inkscape:cy="19.818182"
      inkscape:current-layer="layer1"
      showgrid="true"
      inkscape:document-units="px"
@@ -522,19 +522,27 @@
      inkscape:snap-bbox-midpoints="true"
      inkscape:object-paths="true"
      inkscape:object-nodes="true"
-     inkscape:window-width="1301"
-     inkscape:window-height="744"
-     inkscape:window-x="65"
-     inkscape:window-y="24"
+     inkscape:window-width="3840"
+     inkscape:window-height="2074"
+     inkscape:window-x="3840"
+     inkscape:window-y="49"
      inkscape:window-maximized="1"
-     inkscape:snap-global="true">
+     inkscape:snap-global="true"
+     inkscape:showpageshadow="2"
+     inkscape:pagecheckerboard="0"
+     inkscape:deskcolor="#d1d1d1">
     <inkscape:grid
        type="xygrid"
        id="grid3234"
        empspacing="2"
        visible="true"
        enabled="true"
-       snapvisiblegridlinesonly="true" />
+       snapvisiblegridlinesonly="true"
+       originx="0"
+       originy="0"
+       spacingy="1"
+       spacingx="1"
+       units="px" />
   </sodipodi:namedview>
   <metadata
      id="metadata2821">
@@ -587,7 +595,7 @@
          style="opacity:0.8479996;fill:#ffffff;stroke:none;stroke-width:0.18025407;stroke-opacity:1" />
     </g>
     <rect
-       style="opacity:1;fill:#ff0000;fill-opacity:1;stroke:#ffffff;stroke-width:2.00000024;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1"
+       style="opacity:1;fill:#666666;fill-opacity:1;stroke:#ffffff;stroke-width:2.00000024;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1"
        id="rect4909"
        width="6.1190372"
        height="59.347763"