It's now possible to get/set the selected foreground/background tile
index. Example of use:
app.fgTile = 1 -- the primary tile is '1'
print(app.fgTile) -- this will show '1'
app.fgTile = 0 -- the primery tile is 'no tile'
print(app.fgTile) -- this will show '0'
app.bgTile = 2 -- the secondary tile is '2'
Something interesting is that Image:drawPixel() must ask for a rehash
to the tileset when the tile is modified, maybe we can find a better
way to handle this in the future.
