More visible layer switcher button

[johanlahti]

New option for showing a tooltip when the toggle button is shown:

// Options of LayerSwitcher.js
{
    showTooltip: {Boolean | Number} If non-false, shows a tooltip. If number, sets the time of visibility in ms
}

// lang object of LayerSwitcher.js
_lang: {
        en: {btnShowTooltip: "Click here to open the layer switcher"}},
        sv: {btnShowTooltip: "Klicka här för att visa lagerväljaren"}}
}