Add a timeline indicator editor icon

Follow-up to https://github.com/godotengine/godot/pull/31424.

engine/icons/optimized/icon_timeline_indicator.svg

@@ -0,0 +1 @@
+<svg height="16" viewBox="0 0 16 16" width="16" xmlns="http://www.w3.org/2000/svg"><path d="m3 0h10l-4 4h-2z" fill="#fefefe"/></svg>

engine/icons/svg/icon_timeline_indicator.svg

@@ -0,0 +1,55 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<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:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   height="16"
+   viewBox="0 0 16 16"
+   width="16"
+   version="1.1"
+   id="svg915"
+   sodipodi:docname="icon_timeline_indicator.svg"
+   inkscape:version="0.92.4 (unknown)">
+  <metadata
+     id="metadata921">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs919" />
+  <sodipodi:namedview
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1"
+     objecttolerance="10"
+     gridtolerance="10"
+     guidetolerance="10"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:window-width="2560"
+     inkscape:window-height="1381"
+     id="namedview917"
+     showgrid="false"
+     inkscape:zoom="73.8125"
+     inkscape:cx="8"
+     inkscape:cy="9.6257409"
+     inkscape:window-x="0"
+     inkscape:window-y="30"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg915" />
+  <path
+     d="m3 0h10l-4 4h-2z"
+     fill="#fff"
+     id="path913"
+     style="fill:#fefefe;fill-opacity:1" />
+</svg>