Duplicate palette icon to avoid issues when using the Palette View

The path of the icon URI in the plugin.xml file is using the workspace
path beginning with "src/...". But in the actual bundle, this prefix
doesn't exist. This commit restore the dedicated "icons" folder which
also holds an instance of this file, which was removed with
24daefb.

See #512

org.eclipse.gef/build.properties

@@ -15,6 +15,7 @@ bin.includes = about.*,\
                plugin.xml,\
                plugin.properties,\
                css/,\
+               icons/,\
                .,\
                META-INF/
 source.. = src/

org.eclipse.gef/plugin.xml

@@ -212,7 +212,7 @@
 			point="org.eclipse.ui.views">
 		<view
 				name="%Palette.Label"
-				icon="src/org/eclipse/gef/internal/icons/palette_view.png"
+				icon="icons/palette_view.png"
 				category="org.eclipse.ui"
 				class="org.eclipse.gef.ui.views.palette.PaletteView"
 				id="org.eclipse.gef.ui.palette_view">