[PATCH] LayerSwitcher: implement layer order and preselection

As the current LayerSwitcher.php comments mention:

    // @TODO Add configuration for initial visibility. (Adjust JS accordingly)
    // @TODO Add configuration for ordering?

here is a patch that implements these features.

In the layer switcher control options configuration, layers are presented as a reorderable table: upon rendering, layer labels are listed in the corresponding order.

Each layer is now associated with an initial selected state (preset with the layer's visible option) that determines if the layer is visible or not upon initial rendering.

Options internal structure is updated. Old-style structure is supported via default option values for the sake of compatibility.

Switcher title can be disabled by specifying it as <none>.

Thanks for considering it.