Add icons for different offline modes

[Klumbumbus]

This is useful if they are put in the toolbar.
(Thats the last addition to GUI icons I plan to do for now. They are pretty much complete now.)

[Klumbumbus]

In 17041/josm:

fix #19820 - Add icons for different offline modes

• offline_osm_api, derived from data/imagery/osmbasedmap, original from Ken Vermette, CC BY-SA 3.0 licensed
• offline_josm_website, derived from logo, original from Diamond00744, GPL licensed
• offline_cache_updates, derived from ​https://openclipart.org/detail/25697/architetto-database, PD and CC0 licensed
• offline_certificates, derived from ​https://commons.wikimedia.org/wiki/File:Application-certificate.svg, PD and CC0 licensed
• offline_all, PD and CC0 licensed

[Klumbumbus]

I think I did fine with the java code, but please review :)

[stoecker]

Looks good except that for public functions a "@since ...version..." should be in the javadoc.

[Klumbumbus]

In 17044/josm:

• fix #19796 - Add more optional values to the presets of runway, taxiway, apron and helipad, small fixes (based on patch by gaben)
• see #19820 - Add missing @since