Support for textured background previews

[irydacea]

In practice, Wesnoth sprites are rendered on detailed digitally-painted art backgrounds composited by the game via the terrain graphics system. While striving for sprites to look good on solid colour backgrounds is generally the best idea, sometimes it's worth keeping their practical use into consideration so that they don't blend too much into terrains.

It would therefore be nice to have textured preview background options in addition to the solid colour ones. Wespal does not need to replicate Wesnoth's terrain system 1:1 for this, as the background textures can be either pre-baked (inefficient due to differing window/display sizes and resolutions) or procedurally composited just from Wesnoth's terrain base graphics, which are simple hexagon tiles.

If we went with the second approach, it would probably be best to provide a way to locate Wesnoth's data files so we don't need to ship image files with the Windows build. There could be two options in Settings > General: one to detect Wesnoth's data path from looking at currently-running processes + installed software, and another to let the user manually specify this path.