Select a style's layer in the MapEditor.

Fixes #514.
Add some warning messages if nothing can be done

Add warning messages if:
    * No layers could be selected
    * A selected layer has no selected geometries

Translations included. Also fixed some translations by using '' instead
of '.

Now when multiple styles are selected in the TOC, we add all the
corresponding layers, not just one.
Merge pull request #515 from agouge/514

Fix datasource from selection in the Map Editor when style(s) are selected in the TOC (#514) 

Now creating a datasource from selection in the MapEditor works even when a style is selected in the TOC.

Fixes #514.


In the Map Editor, if you select a layer's style (and not the layer), select some geometries, then try to create a datasource from the selection, then nothing happens.

This is partially related to #124.