GUI: Add missing flags + rework

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

default/icons/flags/pl.svg

@@ -1 +1,45 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="640" height="400" viewBox="0 0 8 5"><path fill="#dc143c" d="M0 0h8v5H0z"/><path fill="#fff" d="M0 0h8v2.5H0z"/></svg>
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   width="900"
+   height="600"
+   viewBox="0 0 11.25 7.5"
+   version="1.1"
+   id="svg6"
+   sodipodi:docname="pl.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"
+     inkscape:snap-page="true"
+     inkscape:zoom="0.41328125"
+     inkscape:cx="180.26465"
+     inkscape:cy="32.665406"
+     inkscape:window-width="1920"
+     inkscape:window-height="1017"
+     inkscape:window-x="-8"
+     inkscape:window-y="-8"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg6" />
+  <path
+     fill="#dc143c"
+     d="M 0,0 H 11.25 V 7.5 H 0 Z"
+     id="path2"
+     style="stroke-width:1.45237" />
+  <path
+     fill="#ffffff"
+     d="M 0,0 H 11.25 V 3.75 H 0 Z"
+     id="path4"
+     style="stroke-width:1.45237" />
+</svg>