All magic behaviour for constructing sprite filenames
has been removed in favour of an explicit Filename
(and TilesetFilenames for tileset-specific sequences)
property.
The previous MiniYaml.Merge implementation interacted
poorly with yaml inheritance, making it complicated
(or impossible) to override certain keys from Defaults.
The new implementation is simpler: If a key is defined
it will be used. If it isn't, the default (if defined)
will be used. Defaults can be masked by making sure
the same key is defined (even with an empty value)
in the sequence.
This also fixes naming within the sequence code to
distinguish between images (a group of sequences),
sequences (defining a specific sprite/animation),
and filenames for a specific sprite/animation.
To prepare them for documentation generation.
Also added descriptions to SpriteSequence implementations and their properties.
Also made a few code style fixes.
