#243 Layer selection in plugins

[michael michaud]

Many plugins have a layer selection combobox.
The way this parameter is initialized is not homogeneous :

• some plugins remember the last selected layer to propose it again the next time the plugin is called (as for other parameters)
• some plugins always propose the layer selected in the TreeLayerNamePanel
  => second option is probably better for layer selection. Plugins behaviour should be homogeneized