Consolidate Layer attributes

[marshallward]

It looks like libtiled now parses all common layer attributes in a single function (getLayerAttributes), which reads Opacity, Visible, OffsetX and OffsetY.

This may also be why Width and Height are now optional in ImageLayer, since these were legacy attributes in the old Tiled Java application that are no longer used.

Someday we may as well move the parsing of those attributes into something like an ILayer method, which would then be inherited by Layer, ObjectLayer and ImageLayer.