Preliminary refactor of SE code and several UI improvements

#### Code refactoring
* LegendsPanel (--> SimpleStyleEditor)
    * Rename to SimpleStyleEditor since this class is actually just the UI for the Simple Style Editor
    * Replace init method with a constructor (It made no sense to have an init but no constructor)
    * Break up long methods into many small methods, easier to understand and maintain
* Get rid of EPLegendHelper; move its only static method into the SimpleStyleEditor class (the only place it is used)

#### User interface improvements
* Replace all occurrences of "edition" (false cognate) with an appropriate English equivalent (#294)
* Delete the layer toolbar (displaying the layer name) and replace it by a layer tag above the legend panel (seems to be more visible and saves some space)
* Introduce a split pane so that the legend tree and the legend panel can be seen more easily (related to #306); resizing seems to work well and the little arrows allow the user so maximize either pane
* Automatically select a legend in the legend tree when it is added and display its dialog (#438)
* Fix slow scrolling (#440)