GUI: Add missing flags + rework

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

default/icons/flags/el.svg

@@ -0,0 +1,50 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   width="900"
+   height="600"
+   viewBox="0 0 40.5 27"
+   version="1.1"
+   id="svg6"
+   sodipodi:docname="el.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="defs10" />
+  <sodipodi:namedview
+     id="namedview8"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:pageshadow="2"
+     inkscape:pageopacity="0.0"
+     inkscape:pagecheckerboard="0"
+     showgrid="false"
+     width="900px"
+     inkscape:snap-page="true"
+     inkscape:zoom="0.44083333"
+     inkscape:cx="624.95274"
+     inkscape:cy="466.16257"
+     inkscape:window-width="1920"
+     inkscape:window-height="1017"
+     inkscape:window-x="-8"
+     inkscape:window-y="-8"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg6" />
+  <rect
+     fill="#0d5eaf"
+     width="40.5"
+     height="27"
+     id="rect2"
+     x="0"
+     y="0"
+     style="stroke-width:1.50001" />
+  <path
+     fill="none"
+     stroke-width="3"
+     stroke="#ffffff"
+     d="M 7.4999998,0 V 16.5 M 0,7.4999998 h 14.999999 m 0,-2.9999999 h 25.5 M 14.999999,10.5 h 25.5 M 0,16.5 H 40.499999 M 0,22.5 h 40.499999"
+     id="path4" />
+</svg>