Rename PLYReader to TrimeshReader

We're going to repurpose this plug-in to read other file formats as well.
Currently it still only registers itself as PLY file reader. But I'm about to change that.

Contributes to issue CURA-6739.

plugins/TrimeshReader/__init__.py

@@ -0,0 +1,21 @@
+# Copyright (c) 2019 Ultimaker
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from . import TrimeshReader
+
+from UM.i18n import i18nCatalog
+i18n_catalog = i18nCatalog("uranium")
+
+
+def getMetaData():
+    return {
+        "mesh_reader": [
+            {
+                "extension": "ply",
+                "description": i18n_catalog.i18nc("@item:inlistbox", "PLY File")
+            }
+        ]
+    }
+
+def register(app):
+    return {"mesh_reader": TrimeshReader.TrimeshReader()}