Allow to specify sample layers to be loaded only by using a configuration file

[mthh]

Following some discussion by email, this PR now allows to specify the sample layers to be loaded in the UI only by using a configuration file.
It also removes many example layer files which weren't used anymore.

If we merge this, the UI will be feeded only by the sample_layers.json file which will now be an Array of Objects, each of them describing a layer to be added in the interface.
Each of these objects must have a path member (the path to the file on the server) and a name member (it will be used to fetch the display name" in translation files with app_page.sample_layer_box.the_given_name or display directly that name in all languages if there is no match in translation files).
These objects might also have optionally a fields_type member (containing an Array of Objects describing the type of each field to force it's display "as it" when the layer will be opened) and a suggested_projection member (containing an Array of two members to describe how to access that suggested projection).

The order of the layers in the "Sample dataset" dialog will now depend directly on the order of the layers described in this sample_layers.json file.

[mthh]

This also closes #41 (and outdates the few instructions given in #41 (comment)).

[mir123]

Thank you! Excellent news, just found out now after emerging from the forest.