Make overlays independet from map

[wellenvogel]

Make overlays independet from map

[W-Geronius]

Detaching overlays from map would also permit implementing enhancement issue #9.

Potentially a 2 step approach:
Option for a gpx overlay that will show symbols in the map ( no interaction)

Another example (extract) as used by nautin.nl to provide up to date navigation marks as an overlay in OpenCPN, where field <sym> refers to externally defined icons

<?xml version="1.0" encoding="ISO-8859-1" standalone="yes" ?>
<gpx
 version="1.1"
 creator="Nopoint - http://www.nautin.nl/"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns="http://www.topografix.com/GPX/1/1"
 xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd">
<metadata>
<name>Alles.gpx</name>
<time>2019-05-06T11:01:12Z</time>
<keywords>1234670</keywords>
</metadata>
<wpt lat="51.68836666666665" lon="4.54565">
<name>1</name>
<sym>227</sym>
<cmt>geel</cmt>
<desc>geel</desc>
<extensions><opencpn:scale_min_max UseScale="true" ScaleMin="200000" ScaleMax="0" /></extensions>
</wpt>
<wpt lat="51.434749999999994" lon="3.636483333333334">
<name>1</name>
<sym>014</sym>
<cmt>groen</cmt>
<desc>groen</desc>
<extensions><opencpn:scale_min_max UseScale="true" ScaleMin="200000" ScaleMax="0" /></extensions>
</wpt>
</gpx>

[wellenvogel]

What you address here is something different.
Anyway this has been on the list for long time...
I will follow this with #77.