[Patch] Layers merge compatibility check should be mutual

[Don-vip]

To enable or not layers merge, [:source/trunk/src/org/openstreetmap/josm/gui/dialogs/LayerListDialog.java LayerListDialog] currently checks only if one layer accepts to be merged with another.

This should be a reciprocal check, as it forbids for example, with the current implementation to define its own layer class extending OsmDataLayer without having it automatically potentially be merged with another OsmDataLayer (this is something a plugin writer would not want).

Please find attached a patch that performs a mutual check.