Topics on the backend that only specified the selectedLayers without activatedLayers
were not added.

Due to previous commit that set the topic layers in one dispatch, we must make
sure that the layer list to set has no duplicate.

…rsing

This should also be done for all actions, but for the time being only added
it to the url parameter related actions.

This new generic parameter gives the possibility to do different actions in the
store plugins depending on the source of the event. This avoid bulky/flaky tests
using local closure flags.

During the startup we had several issues with the topic selections.

- the main issue is that we first loaded the default language and topic at startup
  and change them later by parsing the URL. One of the drawback was that the
  layerconfig and topics would have been loaded twice from the backend and
  that the default topics layer would have been set before the current topic
  layers resulting in a lot of store mutations for nothing.
  Now we use the lang and topic from the url if they are available at app startup.
  In order to do this as quickly as possible we now keep in store only the topic ID
  as current instead of a link to the current topic.
- the topic management plugin was a bit buggy and not clear with flag to do stuff
  in different mutation trigger. Now we make use of the dispatcher which make
  the code much clearer. In this plugin for example we only want to set the layers
  when the topic has been changed by the user and not by the URL parsing. Url
  parsing event should takes layer from the URL layers param.

This fixed some bugs of topics management introduced in later PR commits and
make the startup procedure clearer by clearly separated concerns.

Now the store plugins don't have any more special code for the startup. The
startup is by the application management router plugin which does the application
startup routing.

Also removed duplicated topics management events which slightly improve startup
time.

This is based on vuex docu.

Vuex mutation are costly and sending in a loop a lots of event is not a good
idea. When changing the languages we need to reload the layer config and
reset all active layers in order to have them translated instead of doing this
in a loop we all set them in one event.
This improve greatly performance with long active layer list.

Instead of an arbitrary name for the dispatch value, uses a convention as follow:

- Use the same keyword as the one in the store
- Some exception for URL parameter which uses the url parameter for simplification