Add two spinbox in the layers properties dialog, to define an offset.

Introduce the class LayerPropertiesDialog, from which all layer dialogs
(i.e. ObjectGroupProperties and ImageLayerPropertiesDialog) inherite.
Add an offset attribute to the Layer class, and edit it with the layer dialog.
Save/load the offset attributes to/from the xml file.
Use the layer offset values to draw tiles at the correct position.

Only isometric tile layers are handled at the moment.
Merge remote-tracking branch 'upstream/master' into layeroffset

Conflicts:
	src/tiled/stampbrush.cpp
Merge remote-tracking branch 'louiz/layeroffset' into testing

Conflicts:
	src/tiled/tiled.pro