Plugin editgpx: nonexisting gpx layers to import

[alv]

Steps to reproduce:

• don't have any data layers, but do have one or more gpx layers open, and any other layers
• use editgpx, i.e. let it "import" something, and convert that gpxedit layer to a gpx-layer
• (now you have one more gpx layer)
• keep editgpx tool mode active
• delete the layer that happens to be active; that can be for example a gpx, measurement or wms-layer

Result: editgpx thinks you need to import something and the selection list still contains the gpx layer you just clicked to delete. It can even import that layer you thought you had just deleted, when in fact the layer you wanted to delete is deleted right after you click OK or Cancel.

If you delete a non-active layer, editgpx import doesn't "kick in". (good)

If you have a data layer available, JOSM jumps to select mode after you have selected "convert to gpx layer" (good)