Use GTK 3.14 rtl icon support

https://bugzilla.gnome.org/show_bug.cgi?id=769533

icons/mail-sent-symbolic-rtl.svg

@@ -0,0 +1,53 @@
+<?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"
+   version="1.1"
+   x="0px"
+   y="0px"
+   width="90.476997"
+   height="57.429001"
+   viewBox="0 0 90.476997 57.429001"
+   enable-background="new 0 0 100 100"
+   xml:space="preserve"
+   id="svg2"
+   inkscape:version="0.48.4 r9939"
+   sodipodi:docname="sent-symbolic.svg"><metadata
+     id="metadata12"><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="defs10" /><sodipodi:namedview
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1"
+     objecttolerance="10"
+     gridtolerance="10"
+     guidetolerance="10"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:window-width="769"
+     inkscape:window-height="480"
+     id="namedview8"
+     showgrid="false"
+     fit-margin-top="0"
+     fit-margin-left="0"
+     fit-margin-right="0"
+     fit-margin-bottom="0"
+     inkscape:zoom="2.36"
+     inkscape:cx="45.477"
+     inkscape:cy="27"
+     inkscape:window-x="0"
+     inkscape:window-y="58"
+     inkscape:window-maximized="0"
+     inkscape:current-layer="svg2" /><g
+     id="Captions"
+     transform="matrix(-1,0,0,1,95,-19.571)" /><path
+     style="fill:#bebebe;fill-opacity:1"
+     inkscape:connector-curvature="0"
+     id="path6"
+     d="M 90.477,18.945 69.9,34.93 69.944,57.429 53.537,46.557 59.335,41.043 36.561,57.429 0,0 90.477,18.945 z M 8.117,5.421 58.105,37.347 66.257,51.199 66.13,33.602 8.117,5.421 z" /></svg>