BookMap, PageBrowser: make help_text translatable

Minor wording update.
Add a larger and thicker info.svg for their top left icon.

resources/icons/mdlight/info.svg

@@ -0,0 +1,62 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   version="1.1"
+   width="48"
+   height="48"
+   viewBox="4 4 40 40"
+   enable-background="new 0 0 24.00 24.00"
+   xml:space="preserve"
+   id="svg4"
+   sodipodi:docname="info-i.svg"
+   inkscape:version="1.0.1 (3bc2e813f5, 2020-09-07)"
+   inkscape:export-xdpi="48"
+   inkscape:export-ydpi="48"><sodipodi:namedview
+   pagecolor="#ffffff"
+   bordercolor="#666666"
+   borderopacity="1"
+   objecttolerance="10"
+   gridtolerance="10"
+   guidetolerance="10"
+   inkscape:pageopacity="0"
+   inkscape:pageshadow="2"
+   inkscape:window-width="1914"
+   inkscape:window-height="950"
+   id="namedview8"
+   showgrid="true"
+   inkscape:zoom="1"
+   inkscape:cx="24"
+   inkscape:cy="24"
+   inkscape:window-x="0"
+   inkscape:window-y="108"
+   inkscape:window-maximized="0"
+   inkscape:current-layer="svg4"
+   inkscape:document-rotation="0"><inkscape:grid
+     type="xygrid"
+     id="grid13"
+     originx="0"
+     originy="0" /></sodipodi:namedview><metadata
+   id="metadata10"><rdf:RDF><cc:Work
+       rdf:about=""><dc:format>image/svg+xml</dc:format><dc:type
+         rdf:resource="http://purl.org/dc/dcmitype/StillImage" /><dc:title /></cc:Work></rdf:RDF></metadata><defs
+   id="defs8" />
+<circle
+   style="fill:none;stroke:#000000;stroke-width:3;stroke-opacity:1"
+   id="path929"
+   cx="24"
+   cy="24"
+   r="17" /><path
+   style="fill:none;stroke:#000000;stroke-width:3;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1"
+   d="M 24,34 V 19"
+   id="path840"
+   sodipodi:nodetypes="cc" /><path
+   style="fill:none;stroke:#000000;stroke-width:3;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1"
+   d="M 24,16 V 13"
+   id="path850"
+   sodipodi:nodetypes="cc" /></svg>