Add ability to Anchor Tiles at the certain points

[stefanbeller]

At the moment tiles are anchored to the lower left.
It should be possible to configure the anchoring per tilelayer to be anchored at lower, mid or upper side as well as left mid and right side.

(Moving issue from the old bugtracker https://sourceforge.net/apps/mantisbt/tiled/view.php?id=57)

The anchoring helps at creating different 3D projections with oversized tiles.
(Issue #147 relates to drawing order of tiles, which also helps for different projections.)

[bjorn]

Per tile layer? I'd probably put it with the tileset since it relates to the tile offset setting.

[MandL27]

Bumping this since I've recently encountered a scenario where anchoring to the bottom middle would be a huge help: positioning sprites larger than a single tile. Currently, with the bottom left anchoring, sprites with padding (because larger sprites in that same sheet use the full width) have all of their left-side padding visible. Upscaling such padded sprites becomes impossible because then only the padding is drawn.