composer: Custom icon for "show formatting" button

This replaces the italic icon with a new, custom icon.

icons/format-toolbar-toggle-symbolic.svg

@@ -0,0 +1,76 @@
+<?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"
+   inkscape:version="1.0beta2 (2b71d25d45, 2019-12-03)"
+   sodipodi:docname="format-toolbar-toggle.svg"
+   id="svg4"
+   version="1.1"
+   height="16"
+   width="16">
+  <metadata
+     id="metadata10">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs8" />
+  <sodipodi:namedview
+     inkscape:snap-midpoints="true"
+     inkscape:snap-smooth-nodes="true"
+     inkscape:snap-intersection-paths="true"
+     inkscape:object-paths="true"
+     inkscape:current-layer="svg4"
+     inkscape:window-maximized="1"
+     inkscape:window-y="0"
+     inkscape:window-x="0"
+     inkscape:cy="11.459729"
+     inkscape:cx="2.7295596"
+     inkscape:zoom="34.414009"
+     showgrid="true"
+     id="namedview6"
+     inkscape:window-height="1016"
+     inkscape:window-width="1920"
+     inkscape:pageshadow="2"
+     inkscape:pageopacity="0"
+     guidetolerance="10"
+     gridtolerance="10"
+     objecttolerance="10"
+     borderopacity="1"
+     inkscape:document-rotation="0"
+     bordercolor="#666666"
+     pagecolor="#ffffff">
+    <inkscape:grid
+       id="grid835"
+       type="xygrid" />
+  </sodipodi:namedview>
+  <path
+     inkscape:connector-curvature="0"
+     id="path2"
+     fill="#474747"
+     overflow="visible"
+     font-family="sans-serif"
+     font-weight="400"
+     color="#000"
+     style="line-height:normal;font-variant-ligatures:normal;font-variant-position:normal;font-variant-caps:normal;font-variant-numeric:normal;font-variant-alternates:normal;font-feature-settings:normal;text-indent:0;text-align:start;text-decoration:none;text-decoration-line:none;text-decoration-style:solid;text-decoration-color:#000000;text-transform:none;text-orientation:mixed;shape-padding:0;isolation:auto;mix-blend-mode:normal;marker:none"
+     d="m 4.494,2 v 2 h 6 c 0.428,0 0.607,0.281 0.482,0.732 L 10.625,6 H 5.385 C 3.546,6 2.254,7.343 1.879,8.74 1.691,9.44 1.705,10.217 2.117,10.895 2.529,11.572 3.369,12 4.277,12 h 6.76 L 12.902,5.268 C 13.337,3.698 12.174,2 10.494,2 Z m 0.89,6 H 10.07 L 9.515,10 H 4.277 C 3.879,10 3.868,9.928 3.824,9.855 3.78,9.783 3.73,9.561 3.81,9.26 3.972,8.657 4.54,8 5.385,8 Z" />
+  <rect
+     y="13"
+     x="2"
+     height="1.9999995"
+     width="10"
+     id="rect837"
+     style="fill:#474747;fill-opacity:1;stroke:none;stroke-width:1;stroke-linecap:round;stroke-linejoin:round" />
+</svg>

icons/meson.build

@@ -11,6 +11,7 @@ icon_files = files(
   'format-ordered-list-symbolic.svg',
   'format-ordered-list-symbolic-rtl.svg',
   'format-text-remove-symbolic.svg',
+  'format-toolbar-toggle-symbolic.svg',
   'format-unordered-list-symbolic.svg',
   'format-unordered-list-symbolic-rtl.svg',
   'mail-drafts-symbolic.svg',

ui/composer-widget.ui

@@ -1183,7 +1183,7 @@
                     <child>
                       <object class="GtkImage">
                         <property name="visible">True</property>
-                        <property name="icon_name">format-text-italic-symbolic</property>
+                        <property name="icon_name">format-toolbar-toggle-symbolic</property>
                       </object>
                     </child>
                   </object>