shell: Set the 'navigate' icon to the Explore page

This better represent the metaphor of this page.

src/gnome-software.gresource.xml

@@ -38,4 +38,7 @@
   <file>gtk-style.css</file>
   <file>gtk-style-hc.css</file>
  </gresource>
+ <gresource prefix="/org/gnome/Software/icons/hicolor/scalable/status">
+  <file preprocess="xml-stripblanks">navigate-symbolic.svg</file>
+ </gresource>
 </gresources>

src/gnome-software.ui

@@ -367,7 +367,7 @@
                   <packing>
                     <property name="name">overview</property>
                     <property name="title" translatable="yes" comments="Translators: A label for a button to show all available software.">Explore</property>
-                    <property name="icon-name">system-software-install-symbolic</property>
+                    <property name="icon-name">navigate-symbolic</property>
                   </packing>
                 </child>
                 <child>

src/gs-application.c

@@ -966,6 +966,9 @@ gs_application_startup (GApplication *application)
 	gs_folders_convert ();
 
 	gs_application_update_software_sources_presence (application);
+
+	gtk_icon_theme_add_resource_path (gtk_icon_theme_get_default (),
+					   "/org/gnome/Software/icons/hicolor");
 }
 
 static void

src/navigate-symbolic.svg

+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16">
+    <g fill="#2e3436">
+        <path d="M8.017 1.054c-3.82 0-6.941 3.12-6.941 6.941 0 3.822 3.12 6.942 6.941 6.942 3.822 0 6.942-3.12 6.942-6.942 0-3.82-3.12-6.941-6.942-6.941zm0 1.98a4.944 4.944 0 014.961 4.961 4.944 4.944 0 01-4.96 4.961 4.944 4.944 0 01-4.961-4.96 4.944 4.944 0 014.96-4.961z" 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-line:none;text-decoration-style:solid;text-decoration-color:#000;text-transform:none;text-orientation:mixed;white-space:normal;shape-padding:0;isolation:auto;mix-blend-mode:normal;solid-color:#000;solid-opacity:1;marker:none" color="#000" font-weight="400" font-family="sans-serif" overflow="visible"/>
+        <path d="M5.756 11.655L9.52 8.992l.759-4.656L6.515 7zm2.577-2.79a.956.956 0 01-1.25-.512.956.956 0 01.51-1.251.956.956 0 011.253.512.956.956 0 01-.513 1.25z" style="marker:none" opacity=".35"/>
+        <path d="M9.51 8.998l.01-.006.76-4.655-3.755 2.656.656.44a.956.956 0 01.413-.332.956.956 0 011.251.514.956.956 0 01-.078.884z" style="marker:none"/>
+    </g>
+</svg>