Most of the art/textures/ui/session/icons/mappreviews/ have snake_case filenames, however, a few have capitals, spaces, hyphens, or parentheses, which is inconsistent and potentially problematic. This patch renames those png map previews and updates their corresponding maps/ files accordingly.
Furthermore, the names of the sandbox map previews were not always obvious (e.g. sandbox-themacks.png), therefore this proposal also applies the sandbox_kushites.png naming scheme to the others for consistency.
[EDIT] mappreview renames:
Barcania (3).png → barcania.png
Introductory_Tutorial.png → introductory_tutorial.png
sandbox-thebritons.png → sandbox_britons.png
Sandbox - Carthaginians.png → sandbox_carthaginians.png
sandbox-thegauls.png → sandbox_gauls.png
Sandbox - Iberians.png → sandbox_iberians.png
sandbox-themacks.png → sandbox_macedonians.png
sandbox-themauryas.png → sandbox_mauryas.png
sandbox-thepersians.png → sandbox_persians.png
sandbox-theromans.png → sandbox_romans.png
sandbox-theseleucids.png → sandbox_seleucids.png
survivalofthefittest.png → survival_of_the_fittest.png
The Persian Gates.png → persian_gates.png
Tropical Island.png → tropical_island.png