Merge Tool should have an option to correct UVs

The Merge Tool uses Blender's built in `bpy.ops.mesh.merge` for the case of vertex merging, which has an option to correct UVs. We should use it. Add it as a checkbox in the top bar like the checkbox for "Allow Multi-Merge."

For edge merging we use `bmesh.ops.weld_verts` and I need to do some more reading on what options it has (i.e. whether it, too, has support for UV correction).