GUI: Add missing flags + rework

Added missing flags
Resized to keep a coherent look
Replaced complex flags with simpler versions

default/icons/flags/cn.svg

@@ -1 +1,64 @@
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+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   width="900"
+   height="600"
+   viewBox="0 0 30 20"
+   version="1.1"
+   id="svg17"
+   sodipodi:docname="zh.svg"
+   inkscape:version="1.1.1 (3bf5ae0d25, 2021-09-20)"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+  <sodipodi:namedview
+     id="namedview19"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:pageshadow="2"
+     inkscape:pageopacity="0.0"
+     inkscape:pagecheckerboard="0"
+     showgrid="false"
+     inkscape:zoom="1.1755556"
+     inkscape:cx="449.57467"
+     inkscape:cy="299.85822"
+     inkscape:window-width="1920"
+     inkscape:window-height="1017"
+     inkscape:window-x="-8"
+     inkscape:window-y="-8"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg17" />
+  <defs
+     id="defs3">
+    <path
+       id="a"
+       d="M0-1L.588.809-.952-.309H.952L-.588.809z"
+       fill="#ffde00" />
+  </defs>
+  <path
+     fill="#de2910"
+     d="M0 0h30v20H0z"
+     id="path5" />
+  <path
+     id="use7-3"
+     d="M 5,2 6.764,7.427 2.144,4.073 h 5.712 l -4.62,3.354 z"
+     fill="#ffde00"
+     style="stroke-width:3" />
+  <path
+     id="use9-2"
+     d="M 10.391203,1.0796109 10.224436,2.9744497 9.2447212,1.3429835 10.996895,2.0880366 9.1422116,2.5142698 Z"
+     fill="#ffde00" />
+  <path
+     id="use11-7"
+     d="M 12.717773,3.3040491 11.828759,4.9856794 11.558932,3.1018754 12.884666,4.4684939 11.009924,4.1415915 Z"
+     fill="#ffde00" />
+  <path
+     id="use13-1"
+     d="M 12.93945,6.6576593 11.441779,7.8303512 11.963888,6.000344 12.616812,7.7888929 11.038503,6.7256593 Z"
+     fill="#ffde00" />
+  <path
+     id="use15-8"
+     d="M 10.352936,8.0645443 10.264867,9.9646677 9.2183586,8.3752203 10.999913,9.0469928 9.1644953,9.5497494 Z"
+     fill="#ffde00" />
+</svg>