GUI: Add missing flags + rework

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

default/icons/flags/et.svg

@@ -1 +1,50 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="990" height="630" viewBox="0 0 33 21"><path fill="#FFF" d="M0 0h33v21H0z"/><path d="M0 0h33v14H0z"/><path fill="#0072ce" d="M0 0h33v7H0z"/></svg>
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   width="900"
+   height="600"
+   viewBox="0 0 30 20"
+   version="1.1"
+   id="svg8"
+   sodipodi:docname="et.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="defs12" />
+  <sodipodi:namedview
+     id="namedview10"
+     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="0.75567573"
+     inkscape:cx="516.75604"
+     inkscape:cy="310.31829"
+     inkscape:window-width="1920"
+     inkscape:window-height="1017"
+     inkscape:window-x="-8"
+     inkscape:window-y="-8"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg8" />
+  <path
+     fill="#ffffff"
+     d="M 0,0 H 30 V 20 H 0 Z"
+     id="path2"
+     style="stroke-width:0.930484" />
+  <path
+     d="M 0,0 H 30 V 13.333333 H 0 Z"
+     id="path4"
+     style="stroke-width:0.930484" />
+  <path
+     fill="#0072ce"
+     d="M 0,0 H 30 V 6.6666667 H 0 Z"
+     id="path6"
+     style="stroke-width:0.930484" />
+</svg>