Scripting: Added FileFormat.nameFilter

[bjorn]

Exposes the file name filter used by a file format.

> tiled.mapFormat("tmx").nameFilter
$1 = Tiled map files (*.tmx *.xml)
> tiled.mapFormat("json").nameFilter
$2 = JSON map files (*.tmj *.json)
> tiled.mapFormat("lua").nameFilter
$3 = Lua files (*.lua)