Corentin Noël authored May 14, 2020 and Corentin Noël committed May 15, 2020

This allows to slightly improve the efficiency of each MapSource as downloading a tile from the internet is not the same as doing it from memory or from the filesystem.
Make the ShumateErrorTileSource render the tile for now.

Use G_DEFINE_TYPE_WITH_PRIVATE and G_DECLARE_*_TYPE.

We're starting the version numbering afresh,
so all API is present from the start.

Use the Shumate name for identifiers and symbols.
Also remove left-over docs for the GTK+ widgetery.