Add private MIME DB classes to libbe_build

* This pulls in some more stuff, like libicon and agg which are also
included in libbe_build, now.
* Update a few libbe_build classes and headers needed to get things
building.
* This likely breaks the <build>mimeset build on Haiku.

Add private MIME DB classes to libbe_build

* This pulls in some more stuff, like libicon and agg which are also
included in libbe_build, now.
* Update a few libbe_build classes and headers needed to get things
building.
* This likely breaks the <build>mimeset build on Haiku.