Add support for horizontal and vertical offset in map layers

[vmenezio]

Plugin version 2.3, from Github Godot 3.2.1 Tiled 1.4.3

Issue description Map layer horizontal and vertical offset values are not accessible from imported metadata.

Steps to reproduce

1. Create a tile layer in a Tiled map with offset set to a non-zero value:

   Resulting .tmx file should have a line like this:

   <objectgroup id="4" name="products" offsetx="0" offsety="-8">

2. Try to retrieve the offset data from Godot:

   print("offset available? ", layer.has_meta("offsetx"))
   >>> offset available? false

Sample map or tileset test-offset.zip