AEConvert: Use a more extensible API.

New API is: create a CAE{Fr,To}FloatConv class foo.
The class is to be considered valid iff foo->is_valid is true.
Call foo->convert to make the conversion.
delete foo when done.


This follows from the discussion on #1290.
With this new API, it should be easy to implement AEConvert stuff based on swr/avr.

How to do this is still to decide:
1) a new .cpp file for the swr based converter, implementing the same classes, and chose at build time which file to build
2) #ifdefery in the current AEConvert.cpp

Anyway, a new converter depends on #1290 to be merged first.