WIP: Remove map source chain

This MR contains a couple breaking changes to the file cache format. See 9a90caea.

• Removes ShumateMapSourceChain, ShumateErrorTileSource, and ShumateTileCache.
• Makes ShumateMemoryCache and ShumateFileCache subclasses of GObject instead of ShumateMapSource, and makes ShumateMemoryCache private. The only concrete implementation of ShumateMapSource now is ShumateNetworkTileSource.
• Replaces fill_tile() with get_tile_async(). fill_tile() decoded the image data and filled the tile; get_tile_async() strictly provides the raw data, and decoding is now done by ShumateMapLayer.

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TODO:

• Add refresh_tile_time back to ShumateFileCache
• Add tests for ShumateNetworkTileSource? (will be tricky because we probably don't want tests to depend on an external website)
• Merge ShumateMapSource and ShumateTileSource, since there isn't really a difference now that ShumateTileCache is gone.

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Implements #15 (closed).