GUI: Add missing flags + rework

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

default/icons/flags/be.svg

@@ -0,0 +1,48 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   width="900"
+   height="600"
+   version="1.1"
+   id="svg22255"
+   sodipodi:docname="be.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="defs22259" />
+  <sodipodi:namedview
+     id="namedview22257"
+     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="0.37712362"
+     inkscape:cx="449.45475"
+     inkscape:cy="584.68892"
+     inkscape:window-width="1920"
+     inkscape:window-height="1017"
+     inkscape:window-x="-8"
+     inkscape:window-y="-8"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg22255" />
+  <path
+     fill="#ef3340"
+     d="M 0,0 H 900 V 600 H 0 Z"
+     id="path22249"
+     style="stroke-width:0.877058" />
+  <path
+     fill="#fdda25"
+     d="M 0,0 H 600 V 600 H 0 Z"
+     id="path22251"
+     style="stroke-width:0.877058" />
+  <path
+     d="M 0,0 H 300 V 600 H 0 Z"
+     id="path22253"
+     style="stroke-width:0.877058" />
+</svg>