It's too expensive to fetch the timestamps when the github
server is busy. See Dick Hollenbeck's comments at the top of
github_plugin.cpp for more info.
Also adds some safety to the other caching algorithms after
seeing github_plugin's wild-west usage of the kicad_plugin.
Fixes: lp:1753143
* https://bugs.launchpad.net/kicad/+bug/1753143
comment for GITHUB_PLUGIN which should flow into Doxygen output.
*) Rewrote:
PCB_BASE_FRAME::Save_Module_In_Library(): now uses fp-lib-table and PROPERTIES.
PCB_EDIT_FRAME::ArchiveModulesOnBoard(): now can archive to any writable library type.
PCB_BASE_FRAME::SelectLibrary(): is now generic for selecting a library, not just the active library.