GUI: Add missing flags + rework

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

default/icons/flags/nb.svg

@@ -1 +1,59 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="1100" height="800"><path fill="#ef2b2d" d="M0 0h1100v800H0z"/><path fill="#fff" d="M300 0h200v800H300z"/><path fill="#fff" d="M0 300h1100v200H0z"/><path fill="#002868" d="M350 0h100v800H350z"/><path fill="#002868" d="M0 350h1100v100H0z"/></svg>
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   width="900"
+   height="600"
+   version="1.1"
+   id="svg12"
+   sodipodi:docname="nb.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">
+  <defs
+     id="defs16" />
+  <sodipodi:namedview
+     id="namedview14"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:pageshadow="2"
+     inkscape:pageopacity="0.0"
+     inkscape:pagecheckerboard="0"
+     showgrid="false"
+     inkscape:snap-page="true"
+     inkscape:zoom="0.34005408"
+     inkscape:cx="354.3554"
+     inkscape:cy="42.640277"
+     inkscape:window-width="1920"
+     inkscape:window-height="1017"
+     inkscape:window-x="-8"
+     inkscape:window-y="-8"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg12" />
+  <path
+     fill="#ef2b2d"
+     d="M 0,0 H 900 V 600 H 0 Z"
+     id="path2"
+     style="display:inline;stroke-width:0.783349" />
+  <path
+     fill="#ffffff"
+     d="M 225,0 H 375 V 600 H 225 Z"
+     id="path4"
+     style="stroke-width:0.75" />
+  <path
+     fill="#ffffff"
+     d="M 0,225 H 900 V 375 H 0 Z"
+     id="path6"
+     style="stroke-width:0.783349" />
+  <path
+     fill="#002868"
+     d="m 262.5,0 h 75 v 600 h -75 z"
+     id="path8"
+     style="stroke-width:0.75" />
+  <path
+     fill="#002868"
+     d="m 0,262.5 h 900 v 75 H 0 Z"
+     id="path10"
+     style="stroke-width:0.783349" />
+</svg>