This is the first commit with a simple tilemap editor. Still buggy but
functional in several ways. Several changes were made:
* NewLayer command can receive a tilemap=true to create a new tilemap
layer
* New ToggleTilesMode command added to switch between the palette and
the tileset in the ColorBar (the ColorBar was expanded to show
colors or tilesets with a generic AbstractPaletteViewAdapter)
* All commands to create new layers were moved to Layer >
New... submenu
* There are a new tileset chunk to save tilesets in .aseprite files,
and a new kind of cels to save tilemaps
* Added doc::LayerTilemap, doc::Tileset, etc. and several other types
to handle tilesets/tilemaps in the doc layer.
* Added doc::Grid class with grid specifications that indicates how a
tilemap <-> tileset must be drawn
* Added and expanded cel operations to work with tilemaps and
conversions between regular LayerImage cels <-> LayerTilemap cels
(e.g. copy cels in the timeline between layer types)
