Added possibility to specify a infoWindow content different from the POI hovertext

This adds the feature to return a tuple from a filter function. The first entry of the tuple will be used as the title of the marker, the second will be used as the infowindow-content.
If the return type is not a tuple, but rather a string, the string will be used as both the title and the content. This is the old behaviour.

**Example config (includes manual POIs)**
```python
worlds['world'] = "../Minecraft-Overviewer-Addons/exmaple"

def signFilter(poi):
    if poi['id'] == 'Sign':
        return "\n".join([poi['Text1'], poi['Text2'], poi['Text3'], poi['Text4']])

def townFilter(poi):
    if poi['id'] == 'Town':
        try:
            return (poi['name'], poi['description'])
        except KeyError:
            return poi['name'] + "\n" 


outputdir = "../manualmarkertest"

rendermode = "smooth_lighting"

renders["render1"] = {
    'world': 'world',
    'title': 'exmaple',
    'markers': [dict(name="All signs", filterFunction=signFilter), dict(name="Towns", filterFunction=townFilter)],
    'manualpois':[{'id':'Town', 'x':-259, 'y':64, 'z':150, 'name':'Ponyville', 'description':'A small village'},
                    {'id':'Town','x':-200,'y':64,'z':100,'name':'Fillydelphia'}],
}
```