This will make AsComponent more versatile.

Using some GLib smartness

It belongs there, since we will later use properties of AsMetadata to
adjust the XML output.
Some serialization is still in AsComponent, since it is used as database
storage format.

This is still very basic, e.g. translations are not yet supported.
This feature will be added later.

Improve the section explaining icon types.

This is a legacy feature which no data provider seems to use anymore.

This code is completely untested and needs some review.

This fully implements the new feature as specified in the 0.7.3 spec.
Before we only had basic support for the new icon-cache layout.