Save stage/pentrails as costume (for animations)

Created a new block: "save screenshot as costume named %s"
%s takes the Costume name
Block is created under sensing
Multiple screenshots with the same name have a counter associated with them so that names are not repeated. 
Users can go to the costumes tab and right click on them to export them as png.
All the changes are in objects.js
@virajmahesh yeah, well. I hate regex. I think I'm gonna write a much simpler and shorter function that uses a HOF instead of enigmatic hieroglyphs:

```JavaScript
SpriteMorph.prototype.newCostumeName = function (name) {

    function stemOf(aName) {
        var ix = aName.indexOf('(');
        if (ix < 0) {return aName; }
        return aName.substring(0, ix);
    }

    var stem = stemOf(name),
        similar = this.costumes.asArray().filter(function (eachCostume) {
        return stemOf(eachCostume.name) === stem;
    }).length;

    return stem + (similar ? '(' + (similar + 1) + ')' : '');
};

```

think this could work?