Show available mapdata as another maplayer

[ij0n]

Create a maplayer that contains the rectangular/polyline outlines of all the available mapdata. pick color as gradient between red=old, green=new

for frontend-displaying use the tech from #158 for backend,

• [x] add extents and (Available from accompanying gdal tilestache xml, see #252)
• [x] age/creation data (User entry from chart or if unavailable conversion date with a warning) of maplayers as metadata
• [ ] modify color according to age

[ri0t]

I'd also like to have this functionality for geoobjects